I also had problems with this.
I can mount my nfs shares on other systems either with plain nfs or nfs4. The debian server uses by default the portmap option "-i 127.0.0.1", but it doesn't seem to be a problem for the other clients.

On the N900 the are some problems though:
- The portmap daemon won't ever start because for some reason, the init script detects "pidof portmap" as portmap itself so it says "Already running." To get it to start I had to edit the lauch script commenting that line out.
- After that, mounting the share, with "-t nfs" gives a:
"mount: RPC: Unable to receive; errno = Connection refused"
- Editing the portmap conf on the server and removing the "-i 127.0.0.1" limitation makes the error message change to:
"mount: IP:/ failed, reason given by server: Permission denied"
- Trying to mount with NFSv4 always fails with:
"mount: mounting IP:/ on mountpoint failed: Invalid argument". Lack of kernel support?

Finally, the solution for me:
- hack the portmap init script to force it to launch the daemon
- remove the "-i 127.0.0.1" portmap option on the server side
- mount with IP:/path/from/server/root instead of IP:/path/from/share/root as I do on the rest of the computers

This is my first time playing with nfs, but as far as I've read on the Internet, this is some legacy behavior, isn't it?