The N900 specifically says it is processing the image for upload when in fact no processing should be necessary as its meant to be uploading as-is, which is why I thought it might be the N900 sharing feature at fault. I suppose Flickr might have a maximum upload size for uploading done via the API, I will have to look into that. It would be a bit silly though if uploading via the website has no such limit.
No more than 10MB per photo, and 150MB per video.