If like me you were a little disappointed with the low quality playback from YouTube here's a simple tip to ensure you get the best quality where available.
YouTube over EDGE/3G seems to transfer a lower quality video - well for me it does anyway.
If you click Menu -> Settings -> Wireless Controls -> Wi-Fi.
This will turn on the WiFi radio on your G1 which will then notify you if a Wireless network is in range you can drag down the notifications bar and click to connect.
If you're near a T-Mobile HotSpot or any other Wi-Fi access point you'll get a notification.