So a 15inch MacBookPro would be more than adequate for even for an extended time in the field, maybe 3-4 months? It would probably be the only decent screen I'd be able to use for examining any of the footage captured. That's great if that's the case as it will certainly save me a few £'s!
Cheers, Simon
For some reason when you say "in the field" I have a vision of you charging your batteries via solar power or something, things going bad and you hunting me down to beat me over the head with whatever machine you buy
Joking aside, I guess like many around here, have been to places where things are not quite easy to get, so be ready for the unexpected no matter which way you go (I think that is a photographer/videographers mantra
) Things that you can take for granted, such as an extra cable or extra drive space, can be a nightmare and the thing you don't bring will be the one thing you need. Could be it just happened to me a couple of times. 
So as a matter of course (depending on space permitting/exacty where I will be)
1.) Extra clone of the laptop drive with all repair utilities and other items, incuding disc images if needed for reinstalls of software and drives if you need them. (I will usually carry some of the larger capacity small drives partitioned out to hold these things.)
2.) Cables, and doubles/triples, such as to hook up cameras to computers, drives to hard drives.
3.) Discs of the repair utilities such as Diskwarrior.
4.) Hard drives and tape. Calculate what you need then budget over.
5.) If you use a mouse or trackpad, bring it if you can if it makes things easier to work with.
Anyway the 15 inch would work, but for 3-4 months at some point you may want more screen real estate. I have worked on my 15 inch for trips of 3-4 weeks and it has been fine for capturing HDV off my cameras (not sure which one you are shooting), scrubbing through clips, putting together rough edits, viewing things (stills and video), sending email and the rest. When I get the opportunity though to plug into a external, even a bad one, to move pallettes and then rest I do it though. If it is something where you think you may need to get further along in a project (meaning color correction in the field for a deadline), there are options such as the Matrox to let you use things to monitor more effectively. For video playback from the timeline you can playback the timeline so it is goes to "Main" which should be okay to give you an idea of what you have.
Not sure what your time frame is before the trip, or if you are near an Apple Store or someone who has a 15 inch to try, but if you are able to test some small samples of your footage you have it probably would be worthwhile to do so ahead of time to get a feel of how it will look/handle. Also take a look at the screens, the 15 inch only come in glossy now, while the 17 inch has a non-glossy option. I would guess you are looking at the mid-priced 15 inch due to more graphics card memory? The price (US) difference between the 15 and 17 at that point is $300. But the 17 inch does not have a user replaceable battery.....
If it is a budget concern and there is a choice between a 15 inch with upgraded RAM (and maybe hard drive) or stock 17inch, then I would lean to the 15 inch, with 3rd party (not Apple) upgrades. Another things is that the 17 inch is heavier by about a pound or so. Does not sound like much, but when lugging things around it can add up. When I do bring the 17 inch I am happy with the extra screen size once I get where I am going, and if it is somewhere where it is going to be placed down for a bit of time and not being lugged around I will bring it. But when portability/do not want to be bothered comes into play, the 15 inch is fine. (In fact I have used my old G4 15 inch in the past to edit/watch movies and have been fine with it.)
I guess the bottom line, for me, is that I did pick up the 15 inch (older model) for my dive trips of 1-3 weeks or so and it has been fine. I have not yet upgraded the RAM which I will do (I stil have 2 Gigs in it and would like more RAM.) Would have also liked to have a larger internal HD for virtual memory. Mine is non-glossy which I like, but Sandrah has a glossy iMac which really is not bad to look at. When I get home I do the final work on the tower with calibrated monitor and a couple of displays hooked up. Get used to how things look on the laptop vs you main production machine so you have a sense what things should reasonably look like on the laptop (plus get an idea of the scopes) and you should be okay... I think 