Yeah, six weeks is usually when you'll first see a heartbeat :-)
Check out this site:
http://www.ohiorepromed.com/hcg_levels.htm
There isn't a heartbeat until 6 weeks.
When I was pregnant, we heard the heartbeat at 10 weeks with the doppler. At first it was hard to find, but after a little bit of moving around, we were able to hear it. It was amazing. We were first able to see the hearbeat at about 7 weeks.
Hope this helps...
I didn't see or hear it with a vaginal ultrasound until 6w1d. I also had an ultrasound at 5w3d and 5w6d and couldn't hear it or see it on either one of those. Its pretty common to first hear/see it on an ultrasound between 6-8 weeks.