There’s a bug in Lion that causes all network connections to Connect As the current user. You can’t Disconnect and then get a username/password prompt; if you click Disconnect you’ll briefly see the “Connect As” button flash by. Then you’re back as the current machine account.
I think the bug hits when the username on the Lion machine matches the user name on the remote machine.
There are several workarounds, but this is the easiest for me.
Go to Finder:Go:Connect to Server. Look for something like:
Now put the username you want to use on the remote server in there:
Now you’ll be asked the password.
(Yeah, still have a Lion machine. Old dual USB MacBook with a swapped drive. Won’t die.)