Mine's been working fine all day... as far as I know. Girlfriend's been sending iMessages all day with no complaints or problems according to her. Might just be a localized thing or specific to your phone? Give her the old shut off/turn on, see what happens.
Mine was behaving a little odd the other day here in the uk. It was for some reason struggling to activate after I switched it off to try to correct it. Phone restart didn't help. It then failed to select my number as the primary sending iMessage address/number!