When I first started using iMessage on my iPad2/Wifi it worked fine. Then one day it stopped working for no apparent reason. Then one day I intentionally drained the battery entirely to recalibrate it. And once I recharged, iMessage magically started working fine again. Worked as expected for about 10 days, then just stopped again for no apparent reason. Any ideas?
Does that happen at every WiFi spot you visit, or just at home? If the latter, try a hard reset of the modem/router to clear out the cobwebs.