What book are you reading?

Diet Coke? Uh, no....it's not a plant or an animal :-)
Also why bother with a 'cheat' meal when this is a lifestyle - you will undo all the good work you have done and set off cravings again. Accepting that sometimes we may not be able to eat paleo because the choices aren't available and we won't be 100% compliant all the time (i.e dinner at someones house) is fine but I'm not sure what book you are reading that includes 'cheat' meals. Most people find their desire for unhealthy food diminishes when they start nourishing their bodies with proper food that supplies everything the body needs and they start feeling amazing.
From my own experience I would recommend doing this whole-heartedly and changing your mindset or not doing it all. Sorry if that sounds harsh but if you are eating higher fat (as you should be with this way of eating) then occasionally throwing in sugar and other rubbish then you are creating a recipe for a health disaster.

For some better reading on the subject I'd suggest 'Primal Blueprint' by Mark Sissons (primal allows some dairy) or 'It Starts with Food' by Dallas and Melissa Hartwig who are also behind the Whole30 program which is a great way to kickstart healthy eating. See the sticky at the top of these boards for more good info :-)

I started eating this way in August 2009 before it was "cool" And I used the three "open" meals, and still do on occasion but I don't "need" them anymore. There are a lot of really good reasons to do so. I used/use them for special occasions or meals out. I use them for birthday parties and visiting people that don't "get it". I use them when I just want a friggin piece of real pizza. I came to paleo after three plus years of monthly appointments with a dietitian and exercise physiologist that couldn't figure out why I wasn't losing weight with the "food pyramid" and then "my plate". I was lucky if I lost 1 pound or sometimes less a month.

In my first month of paleo I lost 10 pounds and about 2 inches from just my waist. By January I had lost 30 pounds and over 10 inches from my waist alone (who knows how many cumulative inches.) All the time eating a few open meals a week. Sometimes they were just a day where I had chili with beans, or Friday pizza night where the kids had pizza and I ate eggplant parmesan. For me open meals made the transition easier, especially with family traditions. Then as time went on I didn't need as many open meals, but they helped make this lifestyle more "doable" for me. The better I felt the less I wanted them, but it was/;is still nice to know that this isn't a religion and if I want to "break the rules" it is acceptable. Keeping it to three meals a week helps me not to fall down the slippery slope into eating crap all the time. I have to plan these meals, if they aren't planned then they can't happen.

If you think that they will help you, then do it. It is your life, and your choice. You may find you feel like crap after you eat some things and choose to forever stop eating them. Or you may find they help you stay on track overall like I did.

Cordain's principle of "three open meals" really isn't much different than Sisson's idea of the 80/20 rule.

from that link:

This:

"The 80/20 principle isn’t about “cheating” 20% of the time."

and this:

" It isn’t about only shooting for 80%, and there’s nothing wrong with achieving 100%. If you set out to make your Primal commitment 80%, guess what. It will likely fall below that. If you set out to make your commitment 100%, you’ll probably settle in somewhere between 80-95% based on where you’re at in the PB transition."

Mark is really trying to emphasize that 80/20 is not about planning 'cheat' meals but accepting that you can't be 100% all the time when circumstances are beyond your control. The 20% is more a psychological game so that you don't just give up and go all out when a slip up does happen if you know it's okay to do so occasionally.

Since I've been paleo 2+ years and the inflammation has calmed down, it doesn't kill me to accidentally eat something with grain. But I'm still uncomfortable - and I can't do it more than once every month or two without the pain accelerating. It's not worth it. It takes too long to heal. I prefer not to plan a cheat, since I want to reserve my ability to cope with it for real accidents like hidden ingredients or cross-contamination.

That being said, when I was first changing over to paleo, the first three months were full of cheats both accidental and not, because this kind of gigantic lifestyle change doesn't happen overnight. At least for me it didn't. But the pain was instructive.

I do think that is why Cordain doesn't specifically say "cheat" meals, he says open meals. Although, I personally see nothing wrong with planning to have a food that isn't paleo on occasion. I am one of the few people I know that has lived this lifestyle long term and not become ill from foods that being paleo excludes. Do I have an iron gut? Or is it because I still have stuff that isn't paleo on occasion? Who knows, but I'll take it

I disagree with this. I don't know ANY long term Paleo person (as long as they are eating enough healthy fats, and not over-consuming foods that should be moderated such as nuts and fruit and processed meats etc) that has EVER had negative health problems from this lifestyle. Eliminating grains, legumes, commercial dairy, industrial seed/veggie/soy oils, artificial sweeteners/preservatives/flavourings will not make people sick. Quite the opposite. Iron gut? No, the eliminated foods lead to our gut HEALING and then we can better tolerate our occasional indulgences.

I accept that you have a different opinion; however, I don't understand why you would persist with the Paleo lifestyle if you believe that healthy and necessary foods are excluded from it. Confused.