Problem: I’ve got an iPhone 3G running 3.1.2(7D11) and I use a WLAN network at our office which does not broadcast it’s SSID for obscurity security reasons. Since some version, I guess 3.1, when I get within range of the SSID hidden network it no longer auto connects. Frustrating.
Solution: Apple to fix it in the next version I guess.
“Work around”: Not really a workaround, but a sightly less frustrating way of reconnecting… when in range of the network… recreate the network by entering the SSID and leave out any of the security stuff then click Join. The join will fail (for obvious reasons), but will then “remind” the phone that that network exists and will then connect using the previously configured settings.
Darran Boyd 
I feel you on this one… I’m having the same issues. It’s a serious pain in the ass.