The memory issue is tiny between the 12" PB and the 12" iBook. The both only have one SODIMM slot (the 15/17" PB has two). While Apple states that the most that will fit in an iBook is a 512M module, many places sell 1G SODIMMs that work in iBooks. So the RAM difference is really 1280M vs 1152M (the iBook has 128M on the board, the 12" PB has 256M on the board).
Also the iBook is rated at 6 hours on the battery, the 12" PB is at 5 hours. The PB also has a PCMCIA slot, while the iBook doesn't. Both videocards have 32MB memory, and the PB might be marginally faster, but it'd probably come down to a couple frames if anything in something like UT:2004.
For software, the PB comes with more business related software then the iBook while the iBook comes with games and software you'd more likely use at home. The biggest diffs are the PB comes with OmniGraffle, OmniOutliner, GraphicsConverter, and QuickBooks for Mac New User Edition, and the iBook has Quicken 2004 Deluxe, World Book 2003 Edition, Tony Hawks Pro Skater 4, Deimos Rising, SoundStudio, and Appleworks. The PB is .3 lbs lighter, and like 1/4" thinner.
Ofcourse if the software matters, to buy the iBook software for the PB you'd pay:
Worldbook: $59
AppleWorks: $79
Quicken: $69
THPS4: $29
Sound Studio: $59
Deimios Rising: $20
So, you get $315 worth of extra "home user" software with the iBook. The PB comes with around $200 worth of extra software, but 99% of people with a PB would never use OmniGraffle ($69), OmniOutliner ($20), GraphicConverter ($20), or QuickBooks New User edition, which is a crippled version of QuickBooks Pro that is only available with PB's, but I'd put it's worth at around $90-100.
So, I'd say, unless you need the extra 200Mhz or a PCMCIA card go for the 12" iBook and save yourself a ton of money. I love my 12" iBook I got 2 weeks ago
Brian