Right now it's connected via component to the TV and optical to the Receiver for surround sound. Works great, but my TV has HDMI inputs and what if I want to use that? The 360 has no separate optical out like the PS3. It's part of the MS AV cable. However when using an HDMI cable you don't get an optical out and my receiver doesn't have HDMI inputs because it's from right before HDMI became common. The TV has optical out, but only stereo and of course I want surround. 
 
Is there any easy way to solve this? I don't want/can't buy another receiver just because of HDMI when it's only a few years old and works great otherwise (Yamaha RX-V757)

Should be able to do it like this... 
 
http://support.xbox.com/support/en/us/xbox360/hardware/accessories/av/hdmicable.aspx

The suggestion from @pornstorestiffi should work, but you might also consider replacing your receiver. I recently got an awesome Onkyo receiver with HDMI switching for less than $300. Now, I get 7.1 sound through my HDMI without having to worry about other cables or adapters.

50 bucks for a cable is a bit much...and I can't afford a new receiver. The only money I have for this year is going to be used to buy a PS3 slim and some games that are coming out (Dragon Age for example). 
 
I guess I'll just keep using component for now. The picture is great as it is, I don't think HDMI would make a huge difference. I'm going to get a new receiver at some point though, but that might take some time.

apparently  this should work. 
http://www.youtube.com/watch?v=FptCB_3DOz4