There could be problems with your wireless carrier's network. I know that today I have been experiencing minor issues with sending messages via iMessage, but that has only been today. I've gotten a few errors indicating that the iMessage and/or SMS message could not be sent, but a second attempt proved successful. It's only been happening today so I assume it's a problem either with Verizon Wireless or with Apple. Anyway, you can try going to Settings - General - Reset and then select "Reset Network Settings" to see if that'll help.