Last week someone asked on the Team Foundation Server Forum how the notification that a work item has changed could be modified, so instead of including the link to the TFS built in work item detail ASPX the link could directly point to Team System Web Access.
I've contributed with a possible solution. Neno Loje summarized that information and wrote a nice post that even includes screenshots.
You can read about it here.
Yet the post missed an important bit of information. If you are using the latest version of Team System Web Access (the first version with that name) you should point to http://[TeamSystemWebAccessSite]/wi.aspx?id=[WorkItemID].
This url was introduced in the latest version, the previous one had a problem. If you accessed it, without a valid session things wouldn't worked out as expected. This new url fixes that problem, so it is better to use this one, instead of the one refered in the post.