Um, how about survival?I think we all agree that it's a good idea to put some distance between the earth and oneself once people start toying with nanoweapons and such. If I remember correctly, even Eliezer himself proposed a nanolab in space/on the moon to make sure that the technology (and the species) doesn't get lost in a future conflict/accident. If you consider that to be feasible, then there's no reason to reject the Mars variant of this idea.
> To sustain an industrial civilization you need a
> population of hundreds of thousands of specialists.
Who needs a full-blown civilization? All you need is a colony which can sustain itself until uploading, or at least advanced human-machine integration becomes possible (likely a couple of decades).
> If you want to have
> tech advance, you probably need to increase that to several million people
> (so that you can afford to have a few hundred thousand of them devote their
> lives to research).
The real innovations usually come from a handful of people, the rest does work which can (certainly by the time that this would take place, say 20 or 30 years in the future) be automated to a high degree. Especially if you include weak AI, but even without that. Food etc. would be partially brought along (as with today's space missions), and the rest would be grown on location in automated greenhouses (meat could be grown in vitro). Only a handful of specialized people would be needed for maintainance (this assumes that the scientists wouldn't have any knowledge whatsoever of such matters, which is unlikely).
Note that equipment can be sent to Mars well in advance of the actual mission, so you can have quite a pile when you arrive, and you don't have to drag everything along during the mission. Also, if (in retrospect) it turns out that the earth won't be destroyed after all, the Mars colony can stay in touch and abreast of the latest tech "back home", which can be implemented on the base.