From Linux NFS
NFSv4.1 is really developer-only at this point, but it should mostly work.
On the client side: use the latest kernel possible (at least 2.6.31). Mount with:
mount -tnfs4 -ominorversion=1 server:/export/path/
On the server side:
Follow instructions for exporting nfsv4.
Build and install the most recent nfs-utils possible (at least 1.2.0). Restart nfsd.
Check that /proc/fs/nfsd/versions on server says "+2 +3 +4 +4.1". (The "+4.1" is the important part.)
Checking that 4.1 is actually working: watch some traffic in wireshark. The rpc header should list the program version as "4", and the nfs arguments should list the minorversion as "1". Also, all compounds except the first should start with a "SEQUENCE" operation.