I agree with you, this is a problem that appears to be software.
I do not have -current live media ... but the OpenBSD Project's -current install media can be used for testing.
1. Download /pub/OpenBSD/snapshots/<arch>/cd51.iso from your nearest mirror, burn to CD, and boot.
2. At the "(i)nstall, (u)pgrade, or (s)hell" prompt, type "s" and press Enter.
3. # ifconfig re0
---
If the output from ifconfig looks the same, then the problem still exists. If, however,
you now show a connected Ethernet (and perhaps better than 10mbps/half-duplex), you may check to see if you can obtain a network connection:
4. # dhclient re0
-current is now about a week or two in development beyond what will be 5.1-release, expected to be available on or about 1 May 2012.