While there has been no official confirmation from Microsoft of a connection between the two images (why would they?that I could find), we know that both images are real. For example Disable The People Pane In Outlook 2010 [How-To] has the same silhouette:
This video shows the same thing: Outlook 2010 People Pane.
The mugshot is from 1977 and can be found uncropped in Time (for example):
And here is a better comparison image from OES-UK to show how similar the images are:
Either way, this would not have been the first time that Microsoft used the mugshot. In May 1998, Bill Gates used his mugshot in a speech “as proof that almost anything can be found on the Internet” (here’s the video; it’s 25 minutes in). The Albuquerque Journal quoted Microsoft spokesperson Dean Katz as saying “Bill thought the picture was kind of funny. He looks pretty youthful and shaggy-haired there”.
Brill’s Content in September 1998 reported (in the same issue that showed the mugshot on the cover), however, that Gates obtained the mugshot from New Mexico police.