Hi
Subtitle playback on S-VHS machines is not delibrate. It is simply possible because the extra resolution means the teletext signals (these hold the subtitles) are recorded with enough detail to allow the TV to still decode them on playback from S-VHS tape. It is also possible to browse normal teletext recorded at the time of the TV program, although not always reliably.
DVD Recorders actually strip out the teletext signal completely as this would reduce the quality on the picture if it had to be encoded, so you do not get the teletext subtitles, as they are simply not present in the playback signal.
The reason the recorder keeps the Sky+ subtitles is simply because these are layered over the picture by the digital box, and reach the DVD recorder as a normal TV picture, i.e. the subtitles are decoded and are part of what you are watching, and just get recorded along with the rest of the picture.
Regards
Philip