Ever since switching my Acer Aspire One D150 laptop over to 11.04 full-time, I've noticed that whenever I remove the AC adapter plug from the computer it goes into standby immediately. This happens regardless of how long it's been charging. Some quick searching yielded this Launchpad report, but it's been closed for some time with no solution posted. I've got a simple workaround by setting the 'lid close' event in the power manager to simply turn the screen off. Is there a known fix for this or should I open a new issue?
There is a similar - if not more pertinent - launchpad bug for your laptop model.
Reading through the bug report, it appears that the BIOS on your laptop is a good candidate for further investigation.
The bug report mentions version 1.09a of the BIOS that fixes the issue. Can I suggest you look for the latest BIOS update from your vendor website.