I think these have been around for a bit. The ultimate version is to rig up something like Flo Control. ;)

This has been around for years. However, it is a really great product if you have a cat that needs to get in-and-out. The regular cat/dog doors also let in other animals: cats, and rats, whatever. This device gives your kitty a "key" to wear around her neck. When her head gets near the door, it unlocks itself and lets her in. My cat was reluctant to use it at first, but I just held her and stuck her head through the door a few times. Once her head was inside, she walked right in. After about a dozen trips through, she got the idea. This is a very low-tech item: magnetic reed switches, and 9 volt battery, and a solenoid. However, it is very reliable. You can also set it up so the cat can go out, but can't come back in, or can come in but can't go out. Or you can turn it off entirely, so nothing comes in-or-out. Come to think of it, I never used any of those features. 'just let the cat come in and out at her leisure. The device does get a bit dirty from cat hair etc., and needs to be cleaned about once a year. You can change the battery at the same time. These things do work, and very well.