The following has taken me a while to figure out, but here is the answer and it's hopefully useful to someone else with the same (or similar) problem.
I have a Netgear ReadyNAS appliance which I bought because it there was a very good deal on at the time (buy a unit with a 1TB disk and get another 1TB disk for free). It's sat on my desk and not been doing much as I've had other projects to work on.
I decided to configure it as a backup NAS for some of my other machines, specifically T's HP Pavilion and my Aspire Revo, both running Windows 7. I was interested in backing up documents only, so configured the ReadyNAS to connect to each Windows PC fileshare and pull in the data.
This worked perfectly on T's PC but on mine, the ReadyNAS kept complaining that it could not connect to \\REVO\Users. I checked that T's PC could see the Revo share. It could. It tried it with my Mac and could browse the Revo using Finder. I upgraded the firmware in the ReadyNAS. That made no difference.
I then resorted to running smbtree on the command line to see what was happening. This gave me the following output:
cli_rpc_pipe_open: cli_nt_create failed on pipe \srvsvc to machine REVO. Error was NT_STATUS_ACCESS_DENIED
Okay, this was a clue. I remembered the previous problems I had with Samba (which is also running on the ReadyNAS) and Windows 7. Microsoft had set the default authentication protocol to NTLMv2 which was not supported by older versions of Samba. The workaround was to set the Network Security policy on the Windows 7 box to accept NTLM (v1) instead. I check this on the Revo, but it was setup correctly.
I then tried to run smbclient -L //REVO to list the shares on the Revo. The equivalent command worked fine on T's PC, but I got the following on the Revo:
mac-mini:~$ smbclient -L //REVO
session setup failed: SUCCESS - 0
Time to turn on debugging (appending -d10 to the above command) and compare the output against the two machines. This showed that there was a difference in the authentication protocols being negotiated.
It was at this point, I remembered a forum post that when I read it seemed irrelevant. The post stated that if Microsoft Live Sign-On Assistant was running, this could cause problems as it introduces another authentication protocol "mechToken". Really? An application can break file sharing?
I run Windows Live Mesh on the Revo, which uses the Windows Live Sign-On Assistant. I uninstalled this. The connection worked! I then tested the ReadyNAS connection and... it worked!
I'm not sure where the "fault" lies here but life would be much easier if these protocols were all properly documented and were designed to gracefully fail if the software sees something unfamiliar.