Ok, so while not technically a Mega Man cameo, it is a cameo that relates to Mega Man, as a familiar icon of sorts appears in Bionic Commando Rearmed, the Xbox Live/PlayStation Network remake of the original NES game Bionic Commando, and the prequel to the upcoming next-generation Bionic Commando title for those same consoles. Jeremy "Toastyfrog" Parish caught it, and you can see it in all its glory here.
What other cameos are Capcom sneaking in? Let us know what you might have found, and we'll mention it on the site, hopefully with a new-and-improved Cameos section before long. And if you can provide any screenshots (from new games or old) of Mega Man showing up in such a capacity, so much the better!