Since a few people have asked me about this, I thought I'd pass on this information.
Microsoft has said that the 'Internet Sharing' feature on Windows Phone 7.5 'Mango' will currently only available on new handsets running 'Mango' out-of-the-box for now and that in the future it might be rolled out to a selection of older handset.
That sucks.
However, over on Windows Phone Hacker there's an easy way to get the tethering feature enabled Samsung Focus and the Omnia 7. And it seems simple enough:
If that's no clear enough, there's a video:
WARNING: This is an unofficial hack. Things may well go wrong, and if they do, don't expect support!
(Hat tip to Long Zheng.)