Couple Notes: This will only work on a mac (as it requires the Growl notifications), assumes you are running Tweetie on the desktop and you will have to purchase a $2.99 iphone app called Prowl (but it’s well worth it, read to see why) and you must be running the latest iPhone firmware. A few other caveats, while the technology used to get DM’s & mentions to your iphone is push, Tweetie on your mac does not use push (it polls every few minutes)…So depending on how your iphone is setup for email, you may receive the notice of a DM before the Prowl push notification (thanks to commenter Ryan for pointing this out). Also, i believe this will only work (as far as i know) if the mac you are syndicating notifications from is online 24/7.

Mac Setup
Step 1: Goto the Prowl Website and setup an account / install their Growl notification plugin on your mac.
Step 2: Open the “system preferences” pane on your mac, and access the Growl settings (should be under ‘other’).
Step 3: Click on “display options” and select the Prowl style. Enter your username/password you used to register as per above. You can also configure Prowl to only send notifications to your iphone after your computer goes idle (awesome). Click on verify just to make sure it is connecting properly.
Step 4: Click on “applications” while in the Growl settings. You should see a list of apps. Find Tweetie and click “configure”.
Step 5: If you now click on notifications you should see there are 3 behaviors that trigger Growl notifications in Tweetie: timeline, mentions & DM’s. What you most likely will want to do is either set your DM’s (and/or) your mentions to use the Prowl display style you just installed.
iPhone Setup
Step 1: Head to the app store on your iphone and purchase/install the Prowl app [ iTunes Link ]
Step 2: Boot up Prowl on your iPhone and enter your Prowl username/password.
That’s it! You should now get a push notification anytime Tweetie receives a DM or mention. The cool thing about this is you can use Prowl with any mac applications that trigger Growl notifications (i.e Transmission, Dropbox etc etc)
Update July 8, 2009: Check out OmniGrowl for a great app to handle push notifications for other things like weather, news, traffic, flight status etc…