There have been a lot of frustrated Office 2008 users that have tried to register their copy of Office on the new Mactopia website, but received product invalid error.
Currently, the Mactopia site is broken for this function. "Registration" has nothing to do with the operation of the software - unlike the Activation required for MS Windows software. It's little more than a marketing tool.
If you look at the icon in the Office installer the "Register" button even has a little stamped certificate icon making it appear important. It's as important as signing up for a recipe of the month. You can sign up for the newsletter without registering.
Send Feedback to Microsoft under Help in any Office application to let them know how you feel about this misconception.
Many questions on the newsgroups involve the new Mactopia web site. Links not working. Hard to find previous articles and Kbs. Installers are listed from Newest, but you have previous installers that are required and the interface makes it hard to see what you do need. The new interface for the Microsoft newsgroups creates an interface that is non standard for NNTP. I guess you can tell, I'm a bit frustrated by these issues. Personally, I find the new color scheme and size of fonts hard to use. It's great they are finally working on making the site more user friendly, but when you can't read it, all those improvements go out the window.
In defense of those that are working on the problems, I do know they are working overtime to fix them. IMHO, this is one of those results where someone on high set the release date to coincide with Macworld and the release of Office 2008 regardless of readiness. Users, however, are paying the price with the results.
