I just learned of a new iPhone app from a company called Music in Reach. They have playing info focused on a variety of instruments.
Weston Sprott, second trombonist of the Metropolitan Opera Orchestra has collaborated on the trombone version of the app.
Here's an example video of Weston talking about hand position. You can find other videos like this as well as an ad for the app here on YouTube.
My first impression: it's nice to see that the first people to come out with such a product have done a good job. (of course, maybe they weren't the first..who knows)
Still, a very nice service for only $1.99.