I'm involved with recording a lot of a capella choir performances, and it's not uncommon for the group to drop 1/2 to a full step through the course of a song. I was hoping with Pro Tools' elastic audio feature that I could raise the ending pitch to match the beginning and have it apply the correction throughout the region, but alas, you can only alter the pitch of an entire regions by a static amount. Does anyone know of a way to apply gradual pitch shifts?
Although I highly discourage doing this - EVER - in a live recording, as with most problems there is a suitable solution.
The Pitch Shift plugin that comes with Pro Tools can do cent-based shifts in pitch. You can use automation to gradually increase the pitch as the choir's pitch decreases.
If you are not happy with the quality the pitch shift plugin gets you, you could have a look at the Melodyne plugin by Celemony. Very good plugin, which I used multiple times in pop studio productions on instruments, but it might do fine as well on vocals.
Bounce the vocal, use Audition's pitch bender, save as a wave (watch your sample rates) and import back into tools.
I haven't used Pro Tools, but this is easily done in Adobe Audition using Effects -> Time and Pitch -> Pitch Bender.
Sound Forge has an excellent gradual pitch bend effect.