Hardware:
2x 1.7GHz Xeon (Foster) on a SuperMicro P4DC6 workstation board
RadeonHD 3650 AGP (built by VisionTek)

Radeon board has 1x VGA, 1x DVI, and 1x S-Video ports.  A TV is attached to the S-Video port through a generic S-Video multiplexer, the others are not connected.  TV is a US/NTSC standard definition CRT.

System boots into a VGA text console with no problem.  When attempting "modprobe radeon", the system switches to graphics mode, then prints a backtrace on the framebuffer and hangs (no keyboard response, no ping, ssh dies).  A mostly-solid stripe of green pixel noise can be seen across the bottom 2 or 3 scanlines of the TV.

The setup works when using the fglrx driver.  An NV31+nouveau arrangement also seems to work, so I don't think the motherboard is at fault.  This isn't really a "regression" per-se, as it has never worked in any vendor- or self-compiled kernel I've tried (since about July 2010).