2 better choices than the basic stamp. Netmedia's BasicX is a very well defined basic interpreter chip based on an Atmel microcontroler. I think it's too expensive but, I believe it's a drop in replacement.
http://www.basicx.com/
My current and favorite basic chip is the Kronos robotics Dios chip. It's based on the Pic18c452 which is similar to the FRC chip. The language goes beyond the stamps. There are many built in commands and functions that apply to robotics . Take a look at this site. There is allot there.
http://kronosrobotics.com/xcart/customer/home.php
I've found the Dios language to be very productive for prototyping. It is not a drop in replacement for a stamp. You would have to come up with a carrier board. They also sell some Pic16 chips with a less powerful basic. They're cheap for interpreter chips. X-10, serial I/O and frequency output are all supported by functions. Language is free.