Atmel has released ISP enabled chips in the C51 Family:
AT89S2051 (2K), AT89S4051 (4K) AT89S8253 (12K)
The most famous Programmer for many ISP devices which is
the "PonyProg" does not have any support for them yet.
It would be the most kind of you to build in support for
these devices for ISP Programming either through the
Serial or the Parallel Interface.