This appears to be an issue introduced in WebOS v3.0.4. Others are having the same problem.
Accelerometer Issues after 3.0.4 update
---TRY THIS 1st---
HP recently put out version 3.0.5. I saw nothing in the changelog regarding the accelerometer, but you can see if this fixes the issue.
LAUNCHER-->SETTINGS TAB-->SYSTEM UPDATES
---TRY THIS 2nd---
You can also try running the diagnostic test for the compass. It may not work, since the compass and accelerometer should be two separate modules, but it's the closest test to an accelerometer test.
LAUNCHER-->SETTINGS TAB-->DEVICE INFORMATION.
Once in Device Information, tap on the "Device Info" tab on the upper left of the screen. Select DIAGNOSTICS.
---IF ALL ELSE FAILS---
You can use WebOS Doctor from HP. this link will tell you how to use it. Download the 3.0.2 version from WebOS Internals. That will downgrade your tablet back to 3.0.2 which should resolve the issue.