It has been available in extras-devel since end of November, and I haven't seen gnumeric in any other fremantle repository. So you'll have to enable extras-devel to download gnumeric (usual warnings apply for devel repository).

I have been using this version on my N900 for some months and it runs fine -- no problems so far.

But I agree with Djonma: It would be a good idea to get it into extras-testing and extras eventually.