I've seen/heard variable HDMI lag on my SD DVD recorder, using optical out for the audio side. Is the combined processing of the HDMI signal at player and TV ends going to take the same time all the time? Will it always be complete within the frame period? If not, some variability in the lag is inevitable, presumably rising or falling in chunks of 20ms for a 50Hz TV, as extra frames are dropped or regained. Again, talking about SD only, there's a very definite difference in lag between discs but a less noticeable lag within any particular disc.
If you do a Google search, there's what looks like a good article on the subject on a website called Video/Imaging Design Line. It would appear that we should not hold our breath waiting for a solution!
It's the reduction of this lag which is mostly influencing my decision of which BD player to buy. On the one hand, the Samsung has the best option for connecting to legacy amplifiers, with its re-encode feature on the optical out, while the Sony 350 has an adjustable audio delay. I suppose the Sony 550 has the best of both worlds, as long as an analogue audio connection is practical, but it's not far short of double the price of the Samsung.
Tony