ad-hoc may be a nice add-on for such an application, but chances are that if you're sitting on a table with another n900 user and both of you are on wifi, you're on the same network anyway.

the main issue i see with scp as it is is the user interface. there should be a way to:

• temporarily start an SSH server on the receiving end and advertise it via avahi
• select a file on the sending device, choose "share" and select "wifi" => name of other N900 as visible via avahi
• have the receiving device ask if the owner wants to accept the transfer and where to save the file
• shut down the SSH server again

starting ssh server and shutting it down could be optional... some users might want to keep the server running, anyway. it should still force some user interaction whenever an incoming scp is detected (which ssh doesn't normally do).

a way to achieve this level of control on the receiving side would be to hard-code the path in the scp-send sharing plug-in (like: always copy to /home/user/MyDocs/.receivedviascp), monitor this directory on the receiving device (by the same application that starts/kills the ssh server) and move any incoming file to a directory chosen by the user as soon as the transfer was completed.