I think part of the problem is interference. There are only so many 'channels' for WiFi and it seems that everyone has a router now and I'd estimate for every router that's broadcasting it's SSID there are at least twice as many that are hidden (though detectable). Add to that the amount of noise in the 2.4 Ghz band from WiFi, Bluetooth, microwave ovens (especially older ones that might not be functioning well), and cordless phones and it's a wonder anything can get connected, much less stay connected.
On the other end of things I think the rush to integrate multiple wireless technologies on one chip is affecting their performance, especially when it comes to power saving. HTC in particular seem to be taking a lot ofshortcuts. My 2003 HTC-built HP iPAQ 4150 can pick up my access point over half a km away -- my 2007 HTC TyTn on the other hand has trouble connecting through one wall. If I hold the Tytn a certain way my WiFi performance improves. It is scary to think that the E-M field around my body is stronger than the 'mighty' Tytn.
ops:
As for stable, flexible routers, the Linksys WRT-54G (espeically in it's 54GL version) is topps in my book. The ability to load custom firmwares is invaluable when the current manufactuers firmware is inadequite or buggy.