Whoa this topic is really amazing, finally some valid discussion on WebFlake. I really enjoy these.
What people mostly beleive is that it's safer to use a retail version of a product and that nulled versions are easier to track. However, this is a mistake. I actually beleive that it doesn't really matter what you use. You ought to be caught one day for using non-licensed software.
I think Davlin's list of things is very accurate so i'm going to type the same thing here. The things IPS will most likely look for when they are checking (patrolling) for unlicensed software are removals of copyright text, added copyright text, certain text not being linked back to IPS etc. etc.
So let's think like IPS for a second, how would we make a decision whether to take action against a website running unlicensed products?
How big is the community? Is it big enough to pursue legal action or not? Most likely forums with around a thousand members won't get noticed easily. "Big" communities are also "bigger" on the web. They are easier for people to come by and they get noticed, so naturally bigger sites also have a bigger chance to get noticed by IPS. Let's say you have 50 thousand members and no license, you're sure to get contacted sooner or later. Websites like webflake will be even higher on the priority list (you know why).
If you're a small website, you'll have less of a chance to get noticed by IPS. And probably, by the time you start growing or have grown into a fully developed community, you'll have enough money (raised?) to purchase the license anyway. Which is what most people do. Seriously, spend the money on the license. It'll be worth it in the long run !