Basically I was unable to connect from my PC running Vista to Windows sharing on my Mac -- doing the normal ipaddressusername and supplying username and password failed. Vista seemed convinced that I'd entered an invalid username and/or password, regardless of what I tried. After much searching, I found the solution in this thread on vista64.net's forums.
You need to go into the Administrative Tools and enter the Local Security Policy section. In that section, you need to change the LAN Manager Authentication Level to 'Send LM & NTLM responses.' If you need full details on the steps involved, see the above-linked post. This change will reduce the security level of your Vista installation, as it downgrades the authentication required for Samba connections -- making it the same as Windows XP.
When I made the change as described, I was able to connect to my Mac from Vista.
[robg adds: From some reading on another thread on vista64.com, it seems the problem is that Microsoft has disabled LM and NTLM authentication, which is what OS X's version of Samba uses. Microsoft's preferred solution is to upgrade to Samba 3, which supports NTLMv2, as does Vista. If you happen to be running Vista Home Premium, you won't have the Administrative Tools application (advanced home users don't need to administer their machines, I guess?). The thread I linked to contains a Registry Editor alternative for Home Premium users.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070405070132724