Update (05/07, 22:45): My update was broken, so the update had to be updated. The filter should now catch DMs, too. Believe it or not, I was wrong that Twitter changed the address that direct message notifications come from; it stayed the same. So that part of the filter didn't need to change. All's well that ends well, right?
Update 2 (05/07, 23:05): Well, scratch the new filters. Twitter went back to the old From addresses tonight. Y'all can use the old filters again. What a fast-paced 24 hours, eh? :P I'll leave this post up just for posterity, but please don't try to use these new filters and then complain that they don't work. ;-)
To make things easier for myself, I'll assume that everyone's seen the old filter setup I published at the end of last month.
The old method was quite convenient for those of us with multiple Twitter accounts, because the email addresses in the From headers changed depending on the address associated with each account (after October 30, 2008 and before this afternoon). Now they all come from noreply@twitter.com (as they used to last year), with the account-specific email addresses tucked away in the reply-to headers (which I can't filter on in Gmail, so that sucks).
Not only did the addresses again become uniform, but that was basically the only easy way to tell the difference between my personal account (which has Topify set up) and the others I run (which don't). Now I have to go through several hoops, and the filter string is longer.
Anyway, here's the updated updated filter string; put this all in the "Has the words" field in Gmail's filter settings:
(to:(you@yourdomain.tld) from:(noreply@twitter.com) subject:"is now following you on Twitter") OR from:twitter-dm-you=yourdomain.tld
As before, you@yourdomain.tld is the email address set in your Twitter account settings, the address to which all your notifications are sent.
I won't bother making an XML file for the new filters, because it's only one field. I'll probably leave the old one for posterity—at least until my Google Page Creator account is completely borked by the transition to Google Sites—because it's easier than deleting it and then updating my old post to reflect that.
Just for the record, Twitter, I'm not happy that I'm having to retool my filters this soon. If you want to make me happy again, put back your email headers the way they were last week. kthx