Join Date:Joined: Aug 2009
I had the same issue, after much searching on google I found its a common problem and there's an app called toggle headset that will resolve this. Last time I checked it wasn't in the Marketplace, a quick search on google should lead you to it.
If you have any trouble finding it, let me know and I'll dig out the link for it when I get home.
One word of warning, its unsigned, but it works like a charm for me with no untoward side effects.