It all comes down to what you're trying to achive. AES128 is quite easy to implement on a micro and as for the speed, it takes a 4Mhz PIC16 about 6mS to encrypt or decrypt 16 bytes. As to how long it takes an 8051 - it doesn't take much to port the code and see. Microchip has an app note regarding AES128 encryption - see www.microchip.com

As the others have mentioned, key management is the issue. One pinpad device I saw used an 8051 with external ram to store the key. It was encased in a block of black epoxy and had a light dependant resistor that would remove power to the ram chip if exposed to light. If you choose a micro with internal eeprom that can't be read externally, that would also ease the problem.