We already know that heavily modified and ultra stealthy MH-60 Blackhawks were used in the raid on Osama Bin Laden's compound in Pakistan. The Associated Press has learned that two MH-47 Chinooks were involved as well, and it seems likely that they were also modified to be silent and invisible on radar.
While only two helicopters (the modified Blackhawks) actually landed in the compound in Abbottabad, several more aircraft actually took part in the mission, the AP reports:
Five aircraft flew from Jalalabad, Afghanistan, with three school-bus-size Chinook helicopters landing in a deserted area roughly two-thirds of the way to bin Laden's compound in the Pakistani city of Abbottabad, two of the officials explained. Aboard the Chinooks were two dozen more SEALs, as backup.
As it turns out, having the back-up in place was a good idea, since one of the Blackhawks struck the compound wall and had to be ditched, and a Chinook was called in to get everybody out.
MH-47 Chinooks are big, loud helicopters, pretty much the exact opposite of what you want if you're trying to sneak into Pakistani airspace without being noticed. And no matter how stealthy your Blackhawks are, putting them into a group with conventional Chinooks is going to make the whole force show up on radar. It makes sense, then, to suggest that it's at least a possibility that the MH-47s had also been modified to decrease their noise, radar, and infrared signatures.
Of course, it's also possible that the two helicopter formations were separate, and that by the time the Chinook had to be called in there was much less need for stealth. It's also worth mentioning, though, that the Chinooks are made by Boeing, who helped build the Comanche prototype stealth chopper. While the Comanche never made it into service (as far as we know), some of that technology could have been quietly adapted to other, more familiar platforms.