Sporadic
I was having this same problem yesterday evening (10/26, about 8pm Pacific). If it was just text, the iMessage sent just fine. But if I attached a picture, the sending progress bar would go about half-way, stall, then eventually give me "Message send failure".
My wife (also on a 4s, and both of us at home on same wifi) was able to send just fine (text and/or pictures), but if she sent a picture to ME, it never arrived on my phone and she got "Message send failure".
Started to think something was wrong with my phone. Tried signing out/in of iMessage, no difference. Tried a "reset network settings" and rebooted, didn't help.
So then I picked up my old 3GS ("no service", but iMessage still working via wifi). Tried it and experienced the exact same problem. Short text iMessages went out fine, but trying to send a picture stalled and eventually failed.
So I just waited... Then tried about an hour later. Everything worked perfectly.
My conclusion: Apple's iMessage servers, or at least the one servicing my account, was having some kind of temporary issue that was later fixed. I have also seen some intermittent iCloud email failures (like iCloud saying my account/password was bad when getting mail), so I am thinking Apple just has some "growing pains" with the new data centers and all the new iCloud/iMessage users. Hopefully it will get sorted and become more reliable over the next few weeks.