Do you constantly edit with the viewer or canvas on full rez? Or just for review purposes?
(I will be new to editing in HD, while I do have some experience in FCP)
I'm guessing most editors work on two screens; would one HD screen (or HDTV) together with the 15" suffice for reviewing?
If you're just cutting the images together the MacbookPro is fine using the Apple Prores codec or DVCproHD codec. If you need to monitor the image for color correction then you'll need to get an AJA I/O box ($3000) to connect to a real HD video monitor and if you want to work with uncompressed HD (or just want to color correct at a reasonable speed, then you should consider a Mac Pro tower and digital HD capture card ($2000?) for connecting to the HD video monitor.
I've edited 1080p Prores HQ 10 bit on my MBP without difficulty. The laptop screen is pretty useless for color correction (for movies or stills IMO) and an external computer monitor would be better, but I wouldn't trust it like a real HD production video monitor which will need the I/O or a tower with a capture card.
What format HD will you be shooting?