Yeah, Airport is pretty good about compatibility -- it also supports more wireless security modes than most third-party products do (such as WPA2), adding to its compatibility.
The only limitations of which I know:
- 802.11a networks are out, obviously. I have never personally heard of anyone using one in years, but you never know
- Sometimes, there is inconsistency in the way manufacturers use passwords for WEP, in particular. Of course, they shouldn't use WEP at all, but there's still a lot of hardware floating around without WPA or WPA2 support. But WEP can treat a password as an alphanumeric string (which gets converted to a Hex key) or as the hex key directly. Sometimes, the alphanumeric strings don't work right across brands, and you have to copy in the hex string. I have never seen a network *fail* to work because of this. Just a hassle.
- Any proprietary super-high-speed standard will back down to 802.11g (these are the "108 MBPS" routers that some companies sell) -- they should always work at 802.11g (54 MBPS), but the Airport Extreme cards don't support their high-bandwidth mode. Of course, they are also not inter-company compatible, AFAIK. (That is, the ones from D-Link and Linksys don't play super-high-speed with each other either.)