http://cgi.ebay.com/Original-back-cover-housing-iPhone-4-4G-White-/260646786131
This is the one I got, the cheapest

and I can tell you it's perfectly OEM but no flash diffuser (all backplates currently available are like that). I hope they'll start selling the diffusers soon
edit:
@Frenchjay
The link you poste up looks like it's an original backplate, but my bottom line of writing is slightly different and mine's insulated on the inside just like the original black one I took off (this one you posted doesn't seem to be insulated on the inside).
posted link code: Model A1332 EMC 308B FCC ID: BCG-E2380B IC: 5790-E2380B
my white code: Model A1332 EMC 308A FCC ID: BCG-E2380A IC: 579C-E2380A
my black code: Model A1332 EMC 308B FCC ID: BCG-E2380B IC: 579C-E2380B
Does anyone want to compare their model batch numbers? I'm not sure what's the difference between the A and B....