Update for iPod touch 3rd generation
Yes, you can get iOS 5 on iPod Touch 3G, which includes the iMessage application. However,  make sure that you truly have an iPod Touch 3G and not an iPod Touch 2G, which can only be updated to iOS 4.2.1. According to this link from Wikipedia, iPod Touch 3G can go up to iOS 5.1.1.