The tone at the beginning of a
cassette is called a "head tone" or "sync tone"
(1) and is used by the ultra-fast copy machines at the factory, to signal the start and stop of duplication. The tone is 1,000
Hz, and the standard length for it is 400-500ms. (~ 1/2 sec). On a side note, it can also be generated by some keyboards/mixing hardware, for when you are recording to tape. If the tape deck hears this sync tone, it will start recording ~ 1/2 sec after the start of that tone if its on 'standby', to ensure it's
synchronized with the rest of your hardware.
With
CDR's so popular, I'm not sure how much longer synchronizing a tape deck will be important. =)
While reading up on this so I could answer the question, I came across some interesting data; it seems that some
weather satellites(2) also use a start/stop tone before and after transmittal of an image.
(1) http://members.aol.com/uniquenyc/key1.htm
(2) http://asd-www.larc.nasa.gov/asd_over/glossary/p.html (phase interval)