I think it is fine, but if you can't get both (29fish and twentyninefish) you might want to consider the latter.

It is more likely though that a person would type 29 over twentynine just over the shear size of the word, same goes for things like 103 and such, but thirty could easily be typed.

Sorry if that came off rambly but the size of the number typed will matter and you most definitily don't want something like 2fish.com if you can't get twofish.com but 103fish.com if you can't get it spelled out would be fine it is going to depend on the size of number word typed.

It's worked out alright for 37signals.

True, but I wonder if that is somewhat due to their audience, which is tech savvy. If the audience was the GQ Public, but of say slightly above average to high intelligence, would they be as comfortable thinking that way?

Btw, thirtysevensignals.com is free, that surprised me I guess.

Stuff I have read indicates that, ideally, you should be able to convey it verbally without a lot of explaining. If it isn't clear whether it is "4" or "for" or "fore" in the domain name, you have a problem. I ran into that issue (of not being able to readily convey it verbally) with my first domain name so firsthand experience backs that up (though the issue wasn't a number).