Like many people reading this article, I spend my day with my iPhone in my pocket. When I leave my office, I don't have to think about bringing my iPhone with me, it's already with me. Now if I step out of my office for more than a few seconds, my MacBook Pro automatically locks itself, and when I return, it automatically unlocks itself.
I've been using Airlock for about a week, and think it is pretty much the perfect balance between security and convenience. There is even a manual login option which lets you bypass Airlock using your regular login password (handy in case your iPhone battery dies or you need to use your computer without your iPhone around, possibly because your 7 year-old son has absconded with it to play Super Monkey Ball 2 again).
Airlock will let you automatically open applications when you come into range or go out of range. The functionality is a little limited at the moment - for example, I'd like a way to automatically set my iChat status as "away" when I'm gone and "available" when I'm back. That can be accomplished if you know how to write AppleScript, but it seems like an obvious feature to build in. There are other possible uses for this I can imagine: telling Mail.app to check for mail as soon as I get back, or telling NetNewsWire to resume downloading when I walk out of the room and pause it when I get back, so it will do those functions when my computer is otherwise idle. Again, if those things are possible it would require you to know how to code them in AppleScript.
Airlock will also only "pair" with one device at a time, which can cause problems if several people use the same Mac. For example, if I lock my iMac at home with my iPhone using Airlock and my wife wants to use it, she needs to have my account password, or else she can't get into the computer (there is no way for her to access the regular login panel, which would be another big feature request). However, for the usual use-case of one person and one Mac at an office, Airlock works great.
The webpage also warns that "due to a bug in Mac OS X, Airlock may not be compatible with computers that use wireless keyboards or mice." You can use Airlock for up to three hours without registering, which I would encourage everyone to do. (You can quit it and relaunch it for another three hours also.)
Airlock also recommends making the "activation range slightly larger than it appears necessary." Wireless signal strength isn't an exact science, and interference is possible. I found that sitting across the room from my iMac at home I was much more likely to run into interference than sitting next to my MacBook Pro at my desk at the office.
The dot shows you where your iPhone is located in proximity to Airlock. You can also adjust how often Airlock checks for your iPhone's presence. Checking more often will secure your Mac faster, but will increase the drain on your iPhone battery. There is a slider available to change between "better battery life" and "more responsive."
Fellow TUAW-er Brett Terpstra suggested that "do it yourself" folks might be able to mimic this behavior using RedHand (€ 1.49) and Proximity (free) but added "I found absolutely no method of Bluetooth proximity detection that didn't occasionally boot me out to a lock screen at least once every few hours when I (and my fully-charged, often docked, iPhone) were sitting right next to it." While I was writing this article with my iPhone charging and hooked to my MacBook Pro, Airlock locked me out once. I toggled Bluetooth on my iPhone off/on and Airlock unlocked.
It's not foolproof, but Airlock is one of those great little gems which solves a problem simply and easily, making life a little easier. I hope to see a few improvements (and hope that Mac OS X bug gets fixed for those who use wireless keyboards and mice) but for me it was money I was happy to spend for a little convenience, and so far the "false positives" haven't happened often enough to bother me, especially since I'm not having to log into my MacBook Pro all day long.
Airlock licenses are US$7.77 which allows you to use Airlock on up to three Macs. As mentioned above, you can download and test it for free for up to three hours per launch.