I've kicked the idea around more times than I can count for my 91 supra. There was a rare, dealer-installed option for keyless entry on these cars, and any excess stock was gone by the time I bought the car in 99. I've thought about going aftermarket since it doesn't seem to be all that hard. It's basically a relay to interrupt the factory alarm and then some actuators in the doors, plus a receiver somewhere.
My biggest concern, really, is reliability. My buddy had an aftermarket keyless entry system installed, and it started to go bad and do all sorts of strange things. Random battery drains, windows rolled up/down at random times (it had window control in the system), etc. If nothing else, I don't want that going on in my car. I might give it a shot this summer, though. Seems like it'd be a fun install. Might try remote start too.