I don't think the Wii will necessarily look like crap on your HDTV, but it depends on two things:
1. Your connection between the Wii and the TV
2. Your TV
First and most important is the connection you use. I believe the Wii comes with a composite cable (two audio and one video channel, red/white/yellow). Using these cables, the Wii probably won't look any better than standard definition TV. However, you can buy a set of component cables (three video channels, YPbPr, plus two audio channels). This connection is capable of carrying the full 480p data stream from the Wii. This is step one...
Next, if your TV is newer, it should upscale the 480p signal quite well to it's native display (720p, 768p, or 1080p).
Now, it won't look like the Xbox or PS3 HD output, but the Wii is more about the experience of gameplay rather than output resolution.