I bought this little piece of wonder off of Amazon for $6 including shipping, and it arrived in 3 business days. I'm really glad I bought it! Here's the review I posted on Amazon:
This is truly a neat little device. I often want to play music in my room over my T-Mobile G1, and it's a battle of charge versus play. I have to make sure that if I do play music, I'll have time afterwords to charge back up. Not the case anymore! I'm able to play music and charge my phone at the same time.
The adapter is very compact for what it does; it looks much bigger in the pictures than it actually is. It's about the size of a quarter, and a little longer width-wise. Sound quality is great; compared to the stock adapter that comes with the G1, I found that bass frequencies are carried much more better and with a little more "oompf". I found a lot more power for my system, and it works great.
The build quality is what you would expect; plastic build with very solid adapter ports. I don't expect it to break at all. The phone charges at the same rate of speed as normal, even with playing music. The port that plugs into the phone is the proprietary port that the G1 has, and the "out" USB port is your typical mini-USB port.
Overall, it's a great product, and a must have for all G1 users!