Microsoft was holding back updates to have a staggered update process so only 50% of the phones that were ready to get updates were being triggered. That wall is now down. If the phone is ready, all phones will now receive update notifications. Of course, in the US that still leaves the Dell Venue Pro (which is in the Planning stage) and the Focus v1.4 (which is still in testing). On top of that there are firmware updates coming out.
Next week we’ll start delivering some model-specific “firmware” from handset makers, so don’t be alarmed if you receive an additional update notification on your phone. These updates are designed to activate new Windows Phone 7.5 features or improve your phone’s performance. Not everybody will receive or require one: It all depends on your country, carrier, and phone model.
So it sounds like some radio or drivers but more likely it will unlike internet sharing and visual voicemail based on your carrier.
You can read it all here. Once again, they’re releasing this faster than scheduled. Now if only there were new hardware to go along with it…