My vote is for the IBM. It doesn't even matter what model, though the R5x model is certainly nice. A few thoughts:

1) I currently have an A31 (1.6GHz P4-M, etc). and have had no problems with it. Before that, I had a 770, before that a 760, before that a... I can't even remember. And the number one reason I have stuck with IBM? Durability. I have a bad habit of whacking it into walls when I walk between rooms, or knocking it off the side of the bed on occasion, or catching my foot in the cord and... well, you get the idea.

2) The TrackPoint. Far superior to the touchpad. And cooler looking too.

I would think seriously about the PowerBook. Mine's been a real workhorse. Plus, you don't *have* to run OS X if you don't want to. But it sure is nice.

Mice, and even trackballs (though I get tired faster using them) are Fitts's Law devices (the movement of the cursor is proportional to the movement of the mouse or trackball), which is why I like 'em. Even the touch pad is so, at least to some degree.

The trouble with the Trackpoint, which is why I never touch the silly thing (its generic name is unfit for pubic, er, public discourse), is that how fast the cursor moves depends on how *hard* you press on it.

Maybe never having learned to drive (partly physiological, partly sociological) has something to do with this.

Got to be the IBM, the 1400x1050 screen alone should seal the deal out of the above features, but of course the nipple (I hope that's not too rude John Cowan, but that's what we call it, you might be thinking of something else though...) would've decided it for me ages ago.

so get the IBM...

I use ThinkPad (T30) at work and have a PowerBook (17") at home. Both are great machines, and I highly recommend either of them, but a silvery Mac is a lot more appealing to the left side of my brain, than a classic black of a TP (did you hear IBM plans an abomination of *colored* TPs?). And, indeed, things just work on a PB.

As for TouchPad vs. TrackPoint -- I still can't really get the hang of the former well. The latter acts like a mini-joystick and I like it (although if a lot of mousing around is required, my hands hate me).

I don't know what you mean by "proprietary OS" any more. Seriously. (What's a "non-proprietary OS"? Not Linux, that's for sure.) And in any case, I'm not sure why the OS is such a huge determining factor. The key issues would seem to be (a) availability of the applications that you need to use, (b) usability, and (c) reliability. (b) and (c) are determined by both the hardware and the software; (c) also involves things like service and support.

Having used both PowerBooks and Linux laptops in my role as a mobile worker, I can say unequivocally that the PowerBook was more usable and reliable. All the apps I need run on OS X - or they will once the latest NetBeans gets sorted out. (3.x is fine, though.) And the iLife suite is absolutely the state of the art....

Windows? OS X is _far_ more reliable than Windows XP. I have to reboot my home PC, which runs WinXP, whenever I want to use the fax modem. I imagine I'll have to reinstall the OS to fix this. And this is typical of the problems I've had with Windows over the years. On the other hand, issue (a) - availability of software, specifically Doom 3 and Civilization PTW, some boutique apps - dictates what that system runs.

Get the PowerBook. I'm now on my second and find it does everything I need pretty much out of the box. The only significant issue I've run into so far is lack of J2SE 1.5 support which isn't planned until the next release of the OS.

No question about it: Go with the PowerBook. I switched to a 15" PowerBook after having owned 7 ThinkPads in a row, and clearly this is the most productive computing platform I've ever owned. I still would love to have a TrackPoint instead of the Pad, but you get used to it pretty quickly. The single mouse button is not an issue at all. Given Mac OS X is an Open Source OS at its core (Darwin), and you can run X on top of that, I don't see the issues with proprietary software either.

Get the 17" PowerBook--no question. A full working surface wherever you go. (I got my first 17" after years of 15" PowerBooks, and it's fantastic.)

Heretical thought: useful screen space is defined more in terms of physical width than pixel width, in my experience. (From using a A20p ThinkPad with 15" at 1600x1200 for a couple of years.)

If you must get a ThinkPad, get a T42--much more portable than the R's. (I've got a T41 which is great.) And if you're a masochist, you can now get a 15" T42p with 1600x1200. ;-)