Hurn:
Ultimate does have its uses for the power user, but the average bloke will be well served by the less expensive Home premium and not notice any difference.
This is very much true, the major difference between Ultimate and Home Premium is that you get better networking support with Ultimate (As per Vista Business).
The more completeness of Vista Ultimate networking is by no means a disadvanted to home Premium - the extra it allows is for it to become a member of a Windows Domain and use those Domain services as would be found in the office.
One issue with home premium that you may find is if you run Internet Information Server locally, it will not allow you to do trusted authentication (as its not part of a domain) which means things like SQL Server Reporting Services cannot be installed. But that is only a problem if your into some serious software development.
Not having the domain features of Vista Ultimate is only a hindrance which can be worked around when in an office (map drives manually etc). But at home using a workgroup network is not a problem.
Cheers