The backgrounder app might work (if you hold down the hold button it runs the application in the background) but I've not tested it.
Also, iRealSMS now has a cool feature which allows you to reply to messages in the pop-up SMS notification box. That might work better!
Yep. I use iRealSMS and it has that feature. I love it.
how much does the full version cost? the demo version added like 500 "demo messages" into my inbox was rather annoying
I take it this app is only for jailbroken iPhones? If so, are there any for the non-jailbroken? I didn't see any after a quick look
Yes its for JB phones only. I really doubt you'll see anything like this for not JB phones as apple restrict all those kind of things in the SDK. Just another reason to JB I guess...