Changing the workgroup for your MacSome of this might be new in 10.4. I wish I'd noticed earlier. I'll post an update on all the effects.
If your Mac and Windows computers are in the same workgroup and on the same local network or subnetwork, it’s easier to connect to the Mac from the Windows computer. When the Mac belongs to the same workgroup as the Windows computer, its name appears in the Workgroup Computers panel of My Network Places.
“WORKGROUP” is the default Windows workgroup name for Mac OS X. Using Directory Access, you can change the workgroup name on the Mac to match a workgroup name you are already using.
Open Directory Access (in Applications/Utilities).
If necessary, click the lock icon and type the name and password for an administrator user on your computer.
Double-click SMB/CIFS in the list and type the new workgroup name in the dialog, then click OK.
Update 1/1/2010: In 10.6 Directory Access is no more. This control is now an advanced option in the Network Preference Pane. Open the Pane, click the Advanced button, then go to WINS to specify a Workgroup name.
When I did this in 10.6.2 I could select from a drop down, but my choice didn't "stick". I had to type it in.