As noted above; a mike on the case and second in the headphones. I'm not positive, but would expect that normal mono/stereo headphones plugged in would disable the mic in the case (same as the built in speakers). So unfortunatly you work in 'speakerphone' mode or don't talk.

Finding a replacement might be quite tricky. I have a mono earpiece with mic for my old nokia (1100) phone, but don't know if it's compatable with the N800's stereo connection. I'll test and post my results.

As for sending files, an alternative would be skydrive if you have a hotmail account. http://skydrive.live.com the recipiant would also need a windows live account, or your file would be publically available at least until you remove it. I haven't tested it from the N800 but don't know why it wouldn't work.