My guess on how multiclassing works:
1. Each class has a variety of powers, special abilities, and class-specific talents.
2. A "class training" feat lets you spend a feat to take ONE power, special ability, or talent from another class. Probably from a selected list. Since this is a normal feat slot, you continue to get all the normal special abilities, powers, etc. from leveling up in your primary class.
3. "True" multiclassing will mean taking levels in another class so that you're NOT getting more abilities/etc in your primary class.
4. HOWEVER, most class-specific powers and talents will be tied to character level or prerequisite talents rather than to class level. (So a Fighter10 taking 1 level of wizard would be able to pick from the same spell list as a Wizard11.)
The real question mark here (in my estimation) is how many class abilities will be "static" (like almost all the monk abilities in 3.0e), and how many of them will be based on "talent trees" (like in SWSE). If it's the former, multiclass "dipping" might be very popular.
ANYWAY, my overall guess is that the most popular "dipping" class will be ranger or rogue for two-weapon fighting. I expect not only rangers, rogues, and some fighters to want to dual-wield, but also plenty of clerics, warlords, etc. But this may well be a case of "ranger training" rather than actual multiclassing.
