I'd start with the PrPro SDK, and also look in on that FORUM. Lot of interesting work being done out there.
Good luck,
Hunt
Hi LoopStan,
As Bill said, there is an SDK for Premiere Pro, with many different plug-in types. But for effects plug-ins in particular, you'll see in the Premiere Pro SDK that it is recommended to use the After Effects SDK for cross-product compatibility with both Premiere Pro and After Effects. So if it's effects plug-ins you're interested in, you can grab the AE SDK here: http://www.adobe.com/devnet/aftereffects.html
And the AE SDK forum is here: http://forums.adobe.com/community/aftereffects_general_discussion/aftereffects_sdk That forum covers using the AE SDK to build plug-ins for both AE and PPro.
Enjoy,
Zac