from the official Microsoft Windows Vista web site, then I would say they are safe.
Publisher certificates sometimes do not get updated, although I would say it would be quite embarrassing that Microsoft have failed to certify their own products.
But apart from that I see no problem.
I downloaded several gadgets from the Windows Vista website, however, all but one are published by "Microsoft and partner companies". I managed to install that one and it works great, but whenever I try to install the other gadgets I get a warning saying: "This file does not have a valid digital signature that verifies its publisher. You should only run software from publishers you trust." OK, my question is this: if I previously scan the files with my virus protection program (Microsoft Security Essentials) and they are clean, is it safe to install them on the sidebar?