Attention iPhone developers: get ye to Apple's iPhone Dev Center. Apple released the image for the iPhone OS 2.1 and SDK 2.1 last night. You must be registered to get at these files, which means that you have to agree to an NDA (First rule of iPhone development: don't talk about iPhone development), though for those curious registering for Apple's dev center is free (as noted below, the free account won't get you access to these files).
According to numerous tipsters this .1 release includes the beginnings of the iPhone notification services as well as some improvements in the location awareness area. iPhone users, this is an advanced release of the software so that devs and make sure their apps work, and start whipping up new apps. This won't be available for phone in the wild until some later date (that only Steve and his hairdresser know for sure).
One thing to note: apps created using this SDK/OS combination cannot be sold on the App Store at this time, so I hope you have an extra iPhone or iPod touch laying around.
Update: I was mistaken, this new files aren't available via the free Apple Dev accounts. You have to have a paid membership to get them (though I would imagine they will trickle down to the freebie accounts at some point in the near future), and be a registered iPhone developer.
Update 2: Some folks in the comments are saying they are paid, registered developers and they can't see these files so your mileage may vary.
Thanks to everyone who sent this in.