Is there an iMessage client for OS X?
Solution 1:
Apple has just announced OSX Mountain Lion which incorporates iMessage into the OS in the form of the Messages app. This will work in the same was as iMessage for iOS.
http://www.apple.com/macosx/mountain-lion/
You can currently download the messages beta free from Apple here
Solution 2:
Currently no. In the future it's possible that Apple would come out with an iMessage client, or an add-on for iChat to integrate it (Lion's iChat now includes a plugin system), but at the moment it's a closed Apple system and they haven't provided any access for OS X.