Any time that you encode/decode sound, it will take a little time to do it. Even NICAM is only "Near Instantaneous" (that's what the NI in NICAM stands for). We can tolerate a small delay, but when it gets longer than about 1/4 of a second, we start to notice.
There are two issues here. The first is Sky. Lip-sync problems on Sky channels are largely down to the encoding process, and some channels are a lot worse than others. On ITV3, it is often so bad, that there is often a jump where the sound catches up with the picture ! Cheap and nasty encoding ! Not much you can do about it, except complain, and who complains ?
The second issue is with the decoding equipment. Since not everybody gets the problem across all formats, the problem must lie in the decoder speeds. Just as in PC CPU's, a cheaper chip will not be as fast as a more expensive one, so don't expect the cheapest AV amp to work as well as the most expensive one. Many cheap plasma displays have slower chips than the expensive ones.
If the lip-sync delay is always the same, then a delay device will cure it. However, if the lip-sync delay varies, then there is nothing you can do about it, as far as I'm aware, other than swapping out poorly performing equipment.