ie wifi would work after halt, powering down and back up, but not reboot.
This was happening when /etc/network/interfaces was set up to work with /etc/wpa_supplicant/wpa_supplicant.conf. eg something like this (I also tried wpa-conf instead of wpa-roam).
But it works fine with the simpler-style interfaces file here.
(Most of the examples I found used wpa_supplicant.conf, & it's possible I made a mistake configuring that - though it did work fine after hard resets. Anyway this has taken me quite a while to fix, so I thought I'd write it up. My Pi has 2013-02-09-wheezy-raspbian)