Haha, actually I won't be purchasing any new PowerBook as I sit here in a Psychology lecture typing this on my Uni.s WiFi network on my 12" PB 867Mhz.
I must admit though, the thought HAS ran through my head of eBaying this PB and getting either a new 12" or 15" PB, but I just don't have the utter need to, and here's the main reason: I use this PB 85% of the time on battery, thus it runs at 667Mhz in Power Saver mode...so will the new PB's, thus no real speed gain. The DVI-out on the 12" PB is enticing, but I've got my Apple displays tied up on my PowerMac, so I just decided the new features aren't worth the upgrade...however the NEXT upgrade cycle I'll probably be upgrading if it's major enough.
For those in the market for a PB today though, here's my advice:
If you want a 15", FIRST, look at the maxed out TiBook for only $1,999...that's an insane deal, and the 867 TiBook is only $1,599 in the refurb Special Deals section. The Combo 867 12" PB is only $1,199!
But if you're deadset on a NEW PowerBook, I think the 12" and 15" PBs have seen very nice upgrades. I personally still feel the 17" PB is too big if you plan to be mobile at all; for those who bring their PBs to their office & back only, the 17" size is perfect. But even still, I can't imagine myself right now typing on anything larger than this 12" PB, room is tight in college lecture halls!
If I were getting a new PB, I'd grab the 15" AlBook w/ ComboDrive, simply because I don't need the SuperDrive, however I would buy the 1.25Ghz processor model as it comes w/ AE built-in, backlit keyboard built-in, etc. I feel the greatest value in the NEW PB lineup is the 1.25Ghz 15" AlBook, w/ or w/o SuperDrive is your own personal choice. I'd also buy minimum RAM from Apple & upgrade to at least 1GIG via 3rd party RAM sources.
In your personal case, I'd say "GO FOR IT!" if you absolutely can deal with not having a desktop system, and instead, using your notebook as a desktop replacement...it's a big change, escp. if you don't use an external keyboard/mouse, which I'd highly recommend getting if you decide to go w/ the 15" AlBook by ditching your desktop. In all honesty though, your Cube should yeild a pretty penny on eBay, and be sure to sell the display seperately, and if you sell your 12" PB too, you'll probably have MORE than enough for a 15" AlBook, escp. if you can get an EDU Discount...use that extra cash to bump the RAM, get an external mouse/keyboard, and a nice case/sleeve to protect it. I absolutely love my current 12" PB, it's an amazing machine, and I know you'll love the 15" AlBook as well...let us know what you do!