There was an article somewhere regarding ECC support on AMD desktop hardware, in short unlike intel they take no steps to intentionally disable it and IMC usually supports it, but also motherboard manufacturers take no steps to actually claim support or validate it, though in most cases motherboards do have things necessary for it to work.
Registered memory is a little different in a sense that it, again, requires IMC to support it which often is not the case, but on K8 it is.
Most manuals for K8 boards (s939, s754) i've seen claim "ECC/non-ECC un-buffered memory", though i've tried ECC registered on multiple s939 boards at this point and it works, including actual error reporting.
Since there is no official support the only way to be sure is to try, which is not a big problem given smaller modules are practically free.
From my experience i've also seen much better stability with registered memory - it easily works with 2 sticks per channel, CR1, 200Mhz and relatively lower timings than is possible with unbuffered.
For example on the boards i have i was unable to get 2 unbuffered sticks per channel work reliably with CR1 even at 3-3-3-8 200Mhz(DDR400), without messing with voltage at least. Would not even post most of the time. Which is also confirmed by period-correct discussions - it was seemingly always the case and people just assumed 2 modules per channel require CR2 most of the time.
Given this, larger modules available and low price IMO it is totally worth getting registered memory for K8 builds.
AthlonXP 2200+,ECS K7VTA3 V8.0,1GB,GF FX5900XT 128MB,Audigy 2 ZS
AthlonXP 3200+,Epox EP-8RDA3I,2GB,GF 7600GT 256MB,Audigy 4
Athlon64 x2 4800+,Asus A8N32-SLI Deluxe,4GB,GF 8800GT 1GB,Audigy 4
Core2Duo E8600,ECS G31T-M3,4GB,GF GTX660 2GB,Realtek ALC662