Without getting into OS wars, I'd probably recommend either an Android or iPhone. I personally love the free wireless tethering app (Available on any rooted android phone). I can set my phone in the best possible spot in my house, then be wherever I want with my tablet.
My advice, if you go Android, is to go with a phone that has at least 2.2 and at least 512 MB of ram. Other things to look for is if the phone has an unlocked/locked bootloader, if you choose to hack it like your Gtab. Do your homework, see what works for you!