MBP all the way
I'd do a MacBook Pro all the way man. Here's what I did: Instead of spending another grand on another cpu, perhaps look to MacGurus.com and buy yourself a 4 drive SATA enclosure with 1 or 2 terabytes stuffed inside (+45 dollar sata card for your express 34 slot). Raid them together, and have a fast, fast solid system to keep at home for editing. The throughput will be amazing for photoshop/video/etc. And you'll have lots of space to do those big projects at home. The imac and macbook have no express slot or pci expansion, so those options are out.
I also own a G4 powerbook that's never been in for repairs (knock, knock), though I never met an ibook that hasn't been in due to its plastic casing. The powerbook / mbp's are as solid as solid gets IMO, and with the express slot, you can get yourself a very fast raid system faster than any firewire or firewire raid can get. What's great is: Once you have the enclosure, the drives are hotswappable. So- if you fill up a terabyte with some hefty projects, swap out for another. And those Sata drives aren't really pricey so it's not a bad investment.
So to recap: MBP, great for travel, use on the go.
For the extra bump at home (editing etc) get a SATA RAID system.
By the way- you can do editing without the raid- I'm just sayin- some people get the imac for storage issues, but the RAID will solve this plus give you throughput beyond what the imac can do.
Just a suggestion! Good Luck! Feel free to ask more about the RAID or visit MacGurus.com. I'm their biggest advocate right now mainly because they are quite possibly the friendliest in customer service and helping people find solutions that fit their needs.