UDIMM ECC. Never forget the important part. The average guy that tries server RAM in a standard consumer platform and say that it doesn't POST is just using RDIMM ECC.
And based on my experience, it also works even on consumer CPUs like my Athlon II X4, and ECC itself can be enabled, assumed your Motherboard allows you to do so. So basically, there is not much a reason to get an Opteron, unless they're better specced or cheaper than a similar Athlon II/Phenom II. They aren't even known to be godly overclockers which was the reason why the original S939 Opterons 1xx line was prefered over similarly priced Athlons 64/64 X2.
at all, chinese "AMD only" memory was made exactly from ECC memory, so it's doesn't strange that original ecc modules can run with AMD without problem. But, at all, ECC memory works on some Intel CPU's too. But, Opteron's can handle much more RAM, then consumer CPU's, but there are can be limitations of chipset or non-REC ECC memory doesn't have such capacity.
For example, my working PC made on Socket F with 2 Opteron's 8435 and 128Gb DDR2 ECC REG(running at 1066hz), regular consumer DDR2 modules can't be more then 4Gb, but I have 16 8Gb modules. If AM2 Opteron can handle ECC REG memory in AM2 desktop motherboards, you can install 32Gb memory for DDR2, for example, but no...
P.S:I saw many post's in internet when people write about they have no POST on mobo with ECC REG memory, but it almost always Athlon/Phenom or other CPU, and I cannot find any description of such PC with Opteron where the server memory doesn't work.
P.S: and, otherways, I saw a video, where ddr2 ecc reg memory was running with Xeon 775(771) socket with nforce chipset(NB in chipset, not in CPU).... some question - maybe, with AMD motherboard with nforce, can handle ddr2 ecc reg too? As I know, there are not motherboards with nforce for AM3(AM3+)