well, apple didnt switch for almost 2 years. part of the TiBooks problem is that the titanium surface was painted along with the carbon composite surface.
being a portable, its bound to get knocked around and the TiBooks tended to get unsightly scratches and chips because of this.
another thing about titanium is that it flexes giving the TiBook a flimsy feeling. aluminum is more ridged and flexes less so the new PBooks should feel solid.
this is based on what i know about titanium and aluminum bike frames. aluminum frames beat you up while titanium absorb a lot of shock and vibration. also, if you feel the 12" pBook, it has a lot less flex than the 15" Pbook or 12" iBook.
The anodized surface should be more difficult to scratch. people also had hing problems which have been addressed with the aluminum PBooks. if you want a 15" and can wait a few months, i would. it shouldnt be that long until updated ones are introduced. i know a lot of resellers that are getting rid of the 15" now for around $500 less than apple advertises them. could be sign of things to come.