Though they don't affect every hi-res powerbook, there are documented screen and audio problems which, to date, have been acknowledged by Apple, but not resolved. Getting one would be a crapshoot. The Lo-res PBs don't have those issues. It's a shame too, cause I like the hi-res resolution and the digital audio ports. Don't know if the DDR2 makes much of a difference in performance.
IMHO, get the macbook, or hold out for one you like. If Apple dropped PCMCIA, then it's probably not bringing it back, so you may as well start planning ahead (though there aren't really any Expresscard products out so far, are there?) Your 1Ghz Tibook still has life. I'm fairly certain that many developers are going probably use 1Ghz as a cut off point for minimum requirements for their software.