I've never received a single T-Shirt I've signed up for, at least, not yet.
Seems like some sort of Internet "snipe hunt" joke.
FWIW:
Site Registration
Microsoft for Partners site registration is required for you to receive your free Microsoft Windows XP t-shirt.
Offer good in the United States only until July 31, 2002, or while supplies last. Limit one gift per person. Please allow 6 - 8 weeks for delivery.
This offer is only available to Microsoft Partners who have not previously registered for the Microsoft for Partners Web site. Microsoft employees are not eligible for this offer.
How to register:
1. Create a .NET Passport account. Please use your business e-mail address (xxx@microsoft.com). Do not use your Hotmail® e-mail account.
2. Fill out the individual form.
3. If appropriate, enter the information for your company so that your coworkers can also enjoy the benefits of the Microsoft for Partners Web site.
4. Provide us with your shipping information.
Registering for the Microsoft for Partners Web site will mean that you will automatically be shipped a Windows XP t-shirt. Please expect your t-shirt within 4-6 weeks.