For anyone who wants to order their 3.5mm solution now, take a look at this one from HTC: http://www.expansys-usa.com/p.aspx?i=159613
Itís the page for one listed as being for a Touch Cruise, but thereís a page with one for every HTC phone. Iíve been using it for a few months on my HTC Touch Cruise and itís great. The plug itself is compact, no different in size than a 3.5mm headphone plug -- and stays put (it isn't shown in the photo). It has 2 parts, the cord with the plug at one end, and a microphone with a 3.5mm jack at the other. The other part is the throwaway earbud headphones that you can toss and use your own. No bulky dongle, and makes any favorite pair of headphones into your microphone/stereo headset. Personally, I forget HTC forgot the 3.5mm jack with this one, as it's just a cable, not an annoying adapter.
The pinout of the mini-usb jack on the G1 is the same as all other HTC phones, so at the very least the microphone and stereo audio will work. Some features of the answer-button that may or may not work on the G1: push once to answer; pushing while on a call places the call on hold; 2 quick presses redials the last number; 1 long press activates voice dial. Most likely, the button will mimick whatever functionality is present with the headset that ships with the device.
It's made by HTC, and is very durable. Beware the adapters from Seidio -- they last a couple weeks and then start to give out, audio stutters, fades in/out -- they're not constructed well and within a month or so the ends start to get loose from the wire; and you can't exactly run to Best Buy for replacements.
The budget-shipping rate from MobilePlanet.com should have them to you well before your G1 arrives