I've also just started noticing this problem. DNS will work the first time I establish a network connection after restarting the device, but /tmp/resolv.conf.wlan0 will be empty on subsequent connections.

I've tried manually editing /tmp/resolv.conf.wlan0 after connecting, but changes seem to be immediately overwritten with an empty file. I know that udhcpc creates this file, but apparently something (perhaps udhcpc itself) is overwriting this file even after the connection is established. Restarting udhcpc or networking does not help; the device must be restarted. Any suggestions would be greatly appreciated.

FWIW, I have an N800 running Diablo (5.2008.43-7).