AES-128 Chip Solutions
Having a separate encryption co-processor could be risky. Anyone with a Bus Pirate or logic analyser and physical access can sniff the bus and pluck out the plaintext data.

You can mitigate the risk a bit with BGAs and/or epoxy, but the best solution is likely to be a SoC which combines MCU with AES.

One approach might be to use a SoC ZigBee chip and ignore the radio part, eg. CC2430, or EM250.

Or, you could go with a Stellaris Cortex-M3 (some have AES tables in ROM)

Or, do it in software. I've used Brian Gladman's code before in projects.