While troubleshooting this issue over the weekend, I came across a few new threads at Apple's discussion forums for AirPort and, more specific to my setup, the AirPort Express. In particular, this AirPort Express-related thread contains some theories and potential solutions from other users in a similar boat. Being that it is now Wednesday, I'm happy to say one of them seems to be working well for me so far, though your mileage may obviously vary. Here's a rundown of my personal setup and the remedy that's bringing some sanity back to my wireless world: typically, I roll with WPA2 Personal security, and since these sketchy connection issues began I've been playing with settings like Interference Robustness and adjusting my APE's (AirPort Express) channel, etc., but to no avail.
As it turns out, users in these forums are beginning to suspect wireless security (at least WPA, not sure about WEP, which seems to be making the ill-advised list these days anyway) as part of the problem, so I simply reset my station with no security. For the past three days, this setup has been working like a charm, though it's understandable if no security simply isn't an option for some users. To help keep our network to ourselves, I turned on 'Create a closed network' in the AirPort tab of the AirPort Admin Utility (that's Apple-speak for "hide SSID"), which simply meant I had to type in my network's name manually from the AirPort menubar item - but you should only have to do this once.
Again, YMMV, but there are a few potential solutions in Apple's discussion forums for these increasingly frustrating AirPort connectivity issues. Let's hope we don't have to keep jury-rigging our base stations for long, and Apple can issue a fix so the "it just works" reputation can actually apply to these things again.