AT&T saw this coming from a mile away...I mostly talk to people with Iphones and was going to downgrade my unlimited text plan....once AT&T saw imessages, they eliminated every text plan except for unlimited. Now it's either $20 for unlimited or pay per text, where only 100 total equals $20 at $0.20 per message.
This actually pissed me off, as I was going to downgrade to the $5 or $10 option. So I got a Google Voice number, gave people I know don't have ios that as my new phone number, and started getting my texts free via Google Voice.
The downside was the Google Voice app conversations were kind of wonky, i'd sometimes have to look at the message a couple of times to clear it as "read", and sometimes I'd be typing a reply and all of a sudden the keyboard would go away and I'd lose everything I'd typed.
But, I jailbroke, installed a great tweak called SMS GV Extension, which integrates your Google Voice texts with the native messages app...yes, basically every sms i send from messages app goes from my Google Voice number, and I receive into native messages app as well!
When I send to someone else with ios5, it goes through imessage using my regular number too!
The only downside is I can't get MMS, and I sent myself an MMS from my girlfriends iphone and it looked like it went through on her phone, but I didn't receive. So the person could think they successfully sent them a pic, but you won't get it. I can send MMS from native messages app, but it sends from my normal number. To me, small price to pay to save $20/month.
So, everyone who I talk to with no IOS now has my Google Voice number, all ios will go thru imessage, and I will be telling AT&T where to stick their overpriced sms plan in about a month. In 2 days I've been talking via native messages app (also utilizing bitesms) non-stop, checked my usage and no sms used!