Hi,
I am not sure at this stage as we have tried most things that cover most of the reasons that this does not work.
Some ISP DNs servers have issues reaching the Ppple Servers used for iMessages so we add Public DNS servers which seem to do a better job.
Some people seem to have issues with the IMAgent so we try Killing the process and relaunching Messages
If that does not work we delete the .plist for IMAgent and restart.
If that does not work we try a Safe Boot as this does seem to sort a few things out.
I would also try a PRAM reset.
Shut down the computer.
Restart it holding down Apple/CMD ( or ⌘)+ALT( ⌥)+P+R Keys until you have heard three Start Up Bongs.
This is similar to using Safe Boot and may "Clear" some memory part that Safe Boot does not.
9:28 PM Friday; August 10, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously