The obvious question is what to do next. We could poll all the PCI devices every second or so to see if there's a PME status register that's been set. This would be reasonably cheap (we're going to wake up at least once a second on x86 anyway, so it makes no real difference to power consumption) but would introduce obvious latency in the wakeup path. This would be fine for certain types of situation (you're probably not going to be too sad if your SD reader takes a second longer to notice a card insertion) but not others (if an sdio wireless card generates a wakeup interrupt then we really ought to do something about it in a more sensible timeframe). A second is to force users to pass a boottime argument to enable this at all. That kind of sucks.
The third possibility is limited to a subset of hardware, but does allow the introduction of some kind of elegance into what would otherwise be one of those nightmarish scenarios that makes me consider taking up farming instead. If we can trigger a PME ourselves then we can test whether the line is connected. This doesn't appear to be possible on SDHCI but the spec for firewire makes it look like we can do it there - one of the interrupt sources is the port enable/disable register, and we can hit toggle that ourselves. I haven't actually tested this yet, but if it works that would let us make this determination.
It's depressing that doing anything interesting with power management is still heavily determined by what Microsoft have bothered to implement - to the extent that on some machines (hello, Thinkpads) there are no GPE methods at all for PME signals and you don't get runtime power management at all. The only thing that saves us is that (a) it's pretty hard to be able to screw up stuff that's already all glued into one chip package, so integrated stuff like USB should be fine anyway, and (b) PCIe does this as part of the normal traffic stream so there's no way to get that wrong. Unless you're still missing the GPE methods for the signals (hello, Thinkpads) in which case you get nothing unless the native PCIe signalling works. I suspect that we can always force that, so there's some hope yet.
In summary, then: dispassionate.