To return to your original question:

Personally, I would go for an Apple Powerbook, but that is only so that I would have an up to date Apple to play with (my current one is at OS 8 )

For scientific applications I would guess that there is far more choice of software for a PC, so I guess that is the way for you to go.

I would go for IBM based on the fact that I have not come across one with a problem (apart from battery replacements or upgrades, which I don't count)

Whatever you get I would recommend investing in a cooling pad (I bet you it makes it more stable and last longer )

Toshibas are good, but they have had heating issues in the past "the phantom typist syndrome". This may well have been fixed, and it only seems to happen after a couple of years of regular use, so it is hard to tell if this is still a potential issue.

Dells are OK but I have encountered some compatibility problems with data projectors, both old ones and the latest.

I realise that I have no idea of the sample population I am dealing with, but IBMs are quite popular around here, and, as I said, I haven't had any issues with them.

Alienware is far too exotic for this part of the World, and I have always looked on them as a gamer's machine and would suspect that they might be a little too "videocentric" (Hey! I just invented a new word for ya! ) for a physisist.

Look for a fast processor and fast RAM?

All in all I would suggest IBM

Cheers