Slashdot | Hotmail Blocks Gmail Emails (and Invites):
Emails and invitations sent to Hotmail from Gmail accounts do not bounce, but nor do they arrive in the recipient's Inbox - they vanish mysteriously into the aether. ... It's already well-documented that Yahoo moves Gmail invites into the Bulk Mail folder.
What this means for Hotmail and Yahoo users is that your email has been sabotaged. You do not get all the email sent to you; all you get is the email that Hotmail/Yahoo deem suitable for your eyes. They're playing free and easy with your email. I hope you didn't get a job offer via a Gmail account....
While it's not surprising that these companies are unethical, it is surprising that they are willing to actively sabotage the very service they provide. Hotmail and Yahoo users can never again trust their email, since it's clear that the service provider will delete any email they feel like deleting.
Update - 2004.06.23
A quick test shows that Hotmail filters Gmail emails (which would include Gmail invites) straight into "junk". I'm not surprised, but it's depressing to see just how morally bereft Microsoft really is.