Some do, some don't.
What you need to look for is the "Upconvert" feature. This will take pretty much whatever you send into it into HDMI for the output. You really don't want to take this feature for granted.
A lot of people confuse this with "upscaling", which is where it will take a lower resolution signal (480 from, say, a DVD player) in and send it out at a higher resolution, say 720 or1080. This is nice, but not all receivers do this equally well. But don't fret. All HDTV's do this internally so having it in a receiver is less important than it seems, but usually these two features are bundled together.