On Windows 7, via either the commandline or in eclipse, I was getting this error:
ADB server didn't ACK
failed to start daemon
This prevents, among other things, any loading of apks onto a phone via usb.
In my case, this seemed to be due to a broken instance of adb.exe still running on my pc. This was hogging the default port adb needed. Killing all running instances of adb via task manager and restarting eclipse cleared things up, and all started running fine again.