I use google voice almost exclusively since I live and work in areas with next to no cell service which go to both my phone and on the google voice webpage. Unfortunately with that, you have to tell people to text your google voice phone number instead of your actual phone number, which I'm not sure if that's what you want to do. Otherwise it's been great for me so far and I've been using it for almost it's entire existence.
If you want to use your existing phone number and the Messages app in iOS: Not really possible. I looked for a JB solution and never could find one.
The best solution I have found is to use Google Voice (if it is available in your country). You will have to use a Google Voice app, and you will need to give your friends a different phone number to reach you by.
iMessage on the Mac does this, but as somebody said above it doesn't support regular text messages.
What a killer feature it would be if it did support regular text messages. Same with messages on iPad. Apple could definitely pull some iCloud magic to make that happen.
This would be great, if i get home and plug my phone into my mac, i would love to have it divert all calls and sms to my computer to access. I think is a must needed for iOS.