For a lot of people the software is the same (like me). So there's nothing religious about it. It's just a different vendor. I try not to even think about the OS. Though after using Mac OS X on a more personal level (as opposed to testing work) I'm finding mroe and more annoying things about Windows. And surprisingly many of the really annoying things about Mac OS X are getting addressed in software updates or can be worked around with third-party software.

My first concern about the iBook is the small screen size. It's also a bit heavy. I wouldn't recommend the 12" PowerBook though because of the NV graphics of course. The 9200 graphics in the iBook are a bit slouchy for 3D and will be very out-dated for when the new Panther OS comes out (ie. won't be able to use many of the new shader stuff in CoreImage). Also, the iBooks' external display is by default Mirror-Only. Not extended desktop like on the PB's. You used to be able to hack around this though if you need this functionality.

The 15" PowerBook is very nice but a lot more expensive.

I can wholeheartedly recommend any of the Apple notebooks over a large number of PC products (from Toshiba and HP for instance). Especially if you're after things like 6-pin firewire, bluetooth, 802.11g, etc.. If you want to upgrade memory, it might make more sense to do it from a third party (at least on the PB's this is true).

Some of the new Toshiba models look decent, but I don't know their specs. e-Machines also had some snappy units that looked and felt a lot better than offerings from the bigger companies like Dell and HP.

Really look at what software you'll be running to help figure this all out. Also, Virtual PC as well as MS Remote Desktop help to fill the gap when you absolutely need to run some Windows content and performance isn't the greatest concern.

[EDIT] I just saw Andy's latest reply. Get the iBook. Your usage pattern will be a nice match for an Apple notebook. Especially once you factor in being able to run the nice Apple software like iDVD and iMovie. They're a bit limited, but can easily get you by in a pinch. Then there's Motion (which really likes more graphics horsepower and a crap-load of RAM) and Final Cut (Express or Pro), not to mention DVD Studio Pro. They simply have no comparable equivalents for Windows. For the generic stuff like the Web I now run Firefox and I'll be switching mail on my Windows machine over to Thunderbird so I can use the same client and files on the Mac. For MP3 tag editing I run my Windows box as a Remote Desktop in a window on my PowerBook.

Bruno