I use an app called textie. It's universal and you use an email address to log in and thats who receivers see who its from. I have it on the iPad and iPhone. I use this to text btw me and my girlfriends phone and it works great she gets my texts almost instantly and I receive texts coming from her in a timely manner. I believe there is also a textie app that includes calls for about 99 cents but I'm not 100% sure about that so if anyone can verify please do. Also the textie app that just does text is free.
iPad 2 64gb white wi-fi
iPhone 4 16gb At&t