NIH SIP (NIEHS, bioinformatics) - Applied 28 Feb, Application Complete 13 March, Accepted 16 April. (Email)
I got an email from the PI followed about a day later by email from the coordinator. I didn't apply to any programs outside the NIH. I was getting pretty concerned until I got my acceptance this week!
I emailed a few researchers, and also one contacted me after searching the database. The internship I got was one of the researchers I contacted, though.
Start sending mails to PIs as soon as your application is complete... I got a lot of "sorry, we already picked our summer student" mails from people who would have been some of my top choices. I did get my 3rd choice lab, though, so it worked out okay in the end.