Are you seeing this on eclipse when you plug your Android device to your Ubuntu box?
Serial Number: ??????????
AVD Name: N/A
Target: unknown
State: ??

Here’s the solution:
1. Create a script to fix this next time it happens, let’s call it “android_device_reset” and save it on a folder contained on your $PATH environment variable.
#!/bin/bash
# android_device_reset script
sudo adb kill-server
sudo service udev stop
sudo adb start-server
sudo adb devices
Save it and make it executable
chmod +x android_device_reset
2. Open this file /etc/udev/rules.d/51-android.rules
Make sure it looks something like this
SUBSYSTEMS=="usb", SYSFS{idVendor}=="0bb4", MODE=="0666"
SUBSYSTEMS=="usb", SYSFS{idVendor}=="22b8", MODE=="0666"
Each line represents a different android device. If you have just one, the file should be one line long.
On that example I’ve configured the rules for a Motorola Droid and a Nexus One.
If you need to know the idVendor numbers for your Android device go here
developer.android.com/guide/developing/device.html#VendorIds
3. Whenever the problem happens, just open a terminal and type
android_device_reset
It’ll ask you for your password, only administrative users will be able to execute the script.
Hope this helps.