It's been a regular four-issue sequence, actually:
Boccob in #338 (December 2005). Olidammara in #342 (April 2006, fittingly enough). Pelor in #346 (August 2006). Wee Jas in #350 (December 2006). Heironeous in #354 (April 2007). St. Cuthbert in #358 (August 2007).
The sole exception is Vecna, in #348 (October 2006), for Halloween-themed goodness.
Likewise, the Demonomicons are on a regular four-issue schedule:
Pazuzu in #329 (March 2005). Fraz-Urb'luu in #333 (July 2005). Zuggtmoy in #337 (November 2005). Baphomet in #341 (March 2006). Kostchtchie in #345 (July 2006). Dagon in #349 (November 2006). Malcanthet in #353 (March 2007). Demogorgon in #357 (July 2007).