Hm... there should be a bit of etiquette for both sides:
A DM should not forbid something in the last moment, unless he has a _very_ good reason for this! "I don't like that PrC" is no very good reason. After all, the player has to play the PrC, not you. Even if you have your reasons, look if you can't alter it so it suits your campaign, or takes away the reasons why you forbid it (it won't be a problem to reduce the HD or give him one less good save, or take away some caster levels).
A Player should not confront the DM with a fait accompli! You probably know in advance what you want, so show it to the DM. So he can alter what he doesn't like, say what he hates about the PrC (and you can counterargument!), and if push comes to shove, you have enough time to take another PrC. Also, this gives the DM enough time to plan the introduction of the necessary facts and persons into the campaign: Your plans for becoming an assassin (as with the DMG PrC) will be much easier to accomplish if there's an assassin's guild somewhere. (Although our DM is very liberal about this: We already had two cases where he even allowed changing feats or taking feats and skills before class so one player or the other could qualify for a PrC we didn't have before: That was verdant lord, since MotW came out last, and Elven High Magi, which a friend of mine I know from some forums made, I changed to suit the campaign and the DM changed for the same reason)
Depending on the PrC, one RPG prerequisite or the other are always a good thing: if it's a PrC associated with an organization, you have to meet them, and gain admission. If it's something that focuses on some abilities, like weapon master, deepwood sniper, or the like, you must show that you train the art, or find someone already belonging to the PrC in order to learn some of the secrets.
On the issue of multiclassing: Personally, I think that you don't need more than 3 base classes and 2 prestige classes. And that's extreme already, and 1/2 or 2/1 could be enough already. I'd allow it if it fits the character (and some PrC's are made to be pushed in between, like the 5-level-classes).
Well, as I said, we have a generous DM: he didn't just allow me to take a 2nd PrC, he advised me to (mainle because it really fitted the character): I play a champion of the elven People and of Corellon Larethian. I started with 5 levels of fighter, then one level of wizard, to become a bladesinger after that. When I reached the upper half of the PrC, I started to plan what to do after 10th level. I first choose the bladedancer (which he banned at first cause he wanted no oriental material, but then agreed upon it because it fitted). Towards the end of my bladesinger career (for the time being, cause of epic levels), I discarded that class (after fulfilling all the prerequisites, but that was OK, since they fit my character anyway) and at level-up to 17th (Ftr5/Wiz1/Bld10/and then some), when I just wanted to do the fighter, he adviced me to chose divine champion. And I saw that I unconciously chose my skills to fulfill the prerequisites (Knowledge(religion)), that I gain the powers of the figher and something in addition, and that it really fitted the character. Now I'm Ftr5/Wiz1/Bld10/Chm2 of Corellon Larethian.
Well, story hour's over, on with your regular thread.