MikeD -
I got my first cassette player about 35 years ago - while I'm not suggesting that all the tapes I ever bought or recorded are still playable, the format is still very much supported.
I have a collection of vinyl LPs that's even older and still possible to play - mind you my father's 78s have been a problem for quite some time now!
I believe that the ISO CD format will be the longest-lived of the optical storage media - it's been around for 20+ years already and showing no real signs of disappearing, partly because of the squabbling over the formats for DVD and next generation optical storage.
In the magnetic tape area I agree that you need to be rather careful - over the last 10 years there have been multiple tape formats, many of which are history already.
Perhaps it is no bad thing, due to the problems that magnetic tape has with de-lamination - I was reading recently about the awful problems that organisations have with their sound archives - oxide layer literally crumbling or peeling off the tape. Very sad - a lot of irreplaceable material is already lost.
