I agree - buy an iBook instead of a 12" PB unless you really need to use it in clamshell mode and/or want an external monitor to be attached and not mirror the primary display.
I have a 12" PB, bought in May, and I LOVE it. Absolutely love it. Typing this on it now. But... with the iBook updates that came after that, I'd recommend the 12" iBook unless one of the few differentiating factors of the PBs sway you. As the PB is just my traveling/post-on-the-couch/etc. Mac, there's no reason it couldn't be an iBook. I bought it because of the scrollpad - which I love - but that's now on the iBooks.
Edit: just noticed you'll be adding an external display. So... maybe the PB is right for you. As far as speed? Well, it's hard for me to say, as I work mainly on my dual-2.0 and my Macintel dev box, but when I was in Finland for a week I did Symbian development (Nokia phone OS, also used by others), compiling and building w/the UNIX version on my PB, and it worked great. Not as fast as the PMs, obviously, but fine. And the portability is absolutely fantastic.