Hi Roger,
Congratulations on your decision. I recently made the jump from PC and have hardly touched my PC since.
I have an 12" iBook which I really like, but I will also be looking to get one of the Intel-based PBs when they are released in 2006. My girlfriend has fallen in love with the iBook (it is beautiful after all) and I could do with some extra grunt for when i start working with the bigger D200 files.
I got an iBook because when I compared it to it's PB cousin, it had better battery life, similar weight, more RAM capacity and (at the time I purchased) very similar processor speed. The PB has the advantage of support for higher rez external displays and DVD burner (along with a few other little things) but these come with a really high price penalty. Some of the larger PB's (15 and 17") also have PC card slots and support for even higher rez external monitors (eg Apple 30" display if that floats your boat).
I have occassionally missed an internal DVD burner, but have an external one and portable HDDs I use instead. You will easily be able to use your current external HDD's as backup on the road and no, you won't need to reformat them.
I can't comment on the relative robustness of each product. My iBook has been excellent so far tho and it has already been across Central America and parts of Australia.
In terms of performance with current software, I don't think there is enough in it to justify the price difference between the iBooks and the PBs. I would imagine this is an artifact of the artificial ceiling imposed currently imposed by the G4 speed/heat limitations.
However, my iBook runs CS2 fine and I can't detect any difference in speed compared to a friend's 17" PB (1.33 vs 1.67) - we both have the same amount of RAM. Nikon Capture on the other hand is a dog on macs (even more so than PC). I can't comment on other software. Of course any basic office/web software runs very nicely on any mac.
Now in terms of future software...Although the current PBs are spec'd with the minimum requirements for running Aperture, it is almost certain that the next generation will run it much better. The likely jump in processor speed (via the Intel chips) in the next gen will significantly assist in running Aperture and no doubt better video cards, with support for Core Image, will also enhance performance.
So I guess it comes down to the eternal question. Do you really need it now, or can you afford to wait til next year. Normally, my feeling on this is that you may as well get it now because things will always be on the improve. But in this case, I think macs (well laptops anyway) are on the cusp of making a significant leap in performance with the switch to Intel and if you can afford to wait until next year you may find you can use your 'puter for longer.
Hope this helps.