I got my C550 yesterday, and it says AA01 on the box. Great picture. I don't know what the input lag is, but compared to the Panasonic G10 I returned 2 months ago, I don't notice any more lag when using Game Mode with my 360.
It plays xvid/mkv from USB drive, and I got it to stream divx .avi files from MediaTomb and MiniDLNA linux servers over the network. Currently working on getting it to stream .mkv too.
edit: got it to work with MiniDLNA. Needed to modify the source code. Samsung TVs seem to want a MIME type of x-mkv instead of x-matroska. The code in upnphttp.c seems to do a big check for which device is connecting (based on the HTTP User-Agent string) and do special-case fixes. One of those is for Samsung TVs, but only recognized older models, so I changed it to reflect the one I saw in the debug logs when my C550 tried connecting.
else if(strstrc(p, "46C550", '\
h->req_client = ESamsungTV;
h->reqflags |= FLAG_DLNA;
h->reqflags |= FLAG_NO_RESIZE;
Later on it uses ESamsungTV to do force the mimetype to x-mkv for mkv files. I'm sure this can also be done for MediaTomb in its config file.