OOC: Yes, you may throw a Grapple attack into a Flurry. It's just like starting a Grapple in the middle of a Full Attack (a Flurry is essentially a modified Full Attack). Though if your Grapple is successful, any remaining attacks in the Flurry/Full Attack start suffering -4 attack and other conditions of being in a Grapple (IIUC).
FAQ:
Can a monk make disarm, sunder, and trip attacks during her flurry of blows? What about grapple checks? What about bull rushes, overruns, or other special combat maneuvers?
As long as every attack is made with one of the monk’s special weapons (that is, weapons allowed as part of a flurry), the monk can perform any special attack that takes the place of a normal attack. She’s free to disarm, sunder, trip, and grapple to her heart’s content.
She couldn’t bull rush or overrun (since those don’t use special monk weapons), nor could she aid another (which requires a standard action) or feint (which requires a move action).
You can find the FAQ here:
http://www.wizards.com/default.asp?x=dnd/er/20030221a
Ooh! There's a new FAQ. Well, there was two years ago, I just discovered it now.