Hey guys, before I go any further let me preface this post by saying I'm new on here and its entirely possible that this is already a well known feature of the Streak or of Android phones in general. That said, I did a search & couldnt find any reference to it, so it may be flying under the radar and I thought it worth mentioning.
I've been using my Streak with an ebay car mount for a couple of weeks now and I have run a stereo 3.5mm cable from the headphone out to the aux input of my stereo for music playback. Initially I was still using the phones loudspeaker for calls, but the other day I forgot to press the speaker button when taking a call on the road & I realised that not only was the sound coming through the stereo (nothing suprising there), but the person on the other end could hear me fine, and clearer than with the loudspeaker on!
After some further testing, it seems the Streak detects whether you have a stereo or stereo+mic jack plugged in & when being used with standard headphones it leaves the built in mic on.
People on the other end of the line can hear me fine over the noise of the car (And my car has a very loud exhaust & has had all sound deadening material removed) and without any of the annoying echo you get on cheaper Bluetooth units. This makes for a very cheap, high sound-quality, in-car handsfree. Especially with most aftermarket headunits now coming with Aux In ports. If your stereo doesn't have this input, any of the FM transmitter units would provide a very similar function at minimal cost.
As I said earlier its entirely possible this feature is common knowledge, but I thought I'd post it anyway, just in case it's not.