It's true that, barring Traveller, you get only 2 classes at a time. However, you can swap out which two classes you have "readied" between every single battle, and you never lose what you've gained in previous classes (that is, you can swap back to previous class abilities any time
Actually, if you take a look at the mechanics we've seen, it seems like this may be somewhat possible. Apparently every level you can retrain a feat. Novice Power, Acolyte Power, and Adept Power all trigger off of your multiclass feat. You don't take Novice Power(wizard), you just take Novice Power. This means, in theory, if you take say Arcane Initiate, Novice Power, Acolyte Power, and Adept Power and then at level up swap Student of Battle for Arcane Initiate, those power feats will be giving you warlord powers now instead of wizard powers. This amounts to the ability to switch up your "second class" every single time you level. It's not quite "change class whenever you want", but it's still pretty good. Note though that this is just going off of pre release information, and they may have put something in the rules to prohibit it.
Also, it is unclear so far how giving up your paragon path in order to multiclass works. It may be possible to use that in order to take a third class.