If I upgrade one of my pc's to Windows 7, what implications are there for compatibility with my WHS. Will it still be recognised, and backed up as are the Vista and XP machines?
To clarify what Ian said: that depends on what you mean by "upgrade". If you are doing a true upgrade install from Vista, you should be able to continue to use that as PC as 1 of the 10 already joined to your server. If, however, you do a new install of 7, it will be a "new PC" in terms of being joined to the server (you would then have both your previous install floating around in your server, plus your new 7 install as well).
Also, one little caveat: 7 is not officially supported on WHS yet (that won't happen until PP3 is released). Having said that, it does work.