I just went through the same thing last week and ultimately decided to sell my WiFi on craigslist (for full price) and bought a 3G on 3G launch day. I thought about tethering but there are a few reasons why I decided against that idea. For one, I've played around with jailbreaking my iPhone, and although it works, it's really unstable and I was getting freeze-ups and slowdowns and weird stuff happening, which is to be expected. Also I'm definitely going to want to install OS 4.0 when it comes out. If I did that on my iPhone, then I would lose tethering until the next jailbreak comes out. And also I'm not that keen on breaking AT&T's terms of service and worrying about whether or not they will nail me for it later.
Ultimately I decided it was better in the long run to just go ahead and get the 3G pad. If you really don't mind spending a little extra time, you could probably make a few bucks shipping your WiFi overseas. The resale value on these things is really high right now, so I figured it was a good time to sell the old one and buy a new one. YMMV