Lately I have seen in a number of pizza stores a sign stating that the bracha on the pizza is mezonos. I would like to explain the issues involved.
The gemara in Berachas (32a), which we just learned in Daf Hayomi states that you make a mezonos on פת הבאה בכסנין, however if you are קובע סעודה you make המוציא. What is פת הבאה בכסנין? There is a 3 way dispute in the rishonim. It is either, dough filled with honey or fruit, dough made with honey or fruit juice, or a very thin dough. All 3 opinions are brought down in שו"ע. There is a big dispute in the acharonim whether these opinions disagree lehalacha. Some say that they are just explaining the gemara but in principle they al agree that the gemara is telling you a principle that any bread that is not used as the principle food but rather is used as a desert or something like that is not bread and you make mezonos. Others argue that is these things specifically and only these things and therefore any thing that doesn't meet the specific criteria above is considered bread.
Pizza today is eaten as the main food of a meal and you are certainly קובע סעודה on it. Therefore, even if the dough technically meets the requirements of פת הבאה בכסנין it doesn't matter you still need to say hamotzi and bench. I don't see how you can say that someone who eats a meal of 2 or 3 slices of pizza doesn't have to bench and therefore these signs would seem to be misleading.
III. Other issues
How do you measure if you are קובע סעודה? Some poskim hold that the shiur of שביעה to be mechayev in benching min hatorah is not just the bread but the whole meal counts as well. Based on this some want to say that if you eat a whole meal and then at the end you have a piece of cake you need to bench because it is all counted and it is like you were קובע סעודה on the cake. In fact R' Moshe seems to hold this way (Orach Chayim chelek 3 siman 32). This is a tremendous chumra nowadays because many times we don't wash and then eat cake at the end of the meal. The Chazon Ish (Sima 34 sif katan 5) argues.
In short, eating cake is very problematic, it is not clear what beracha you need to make.