Android has a really nice security procedure that forces the user to enter his/her Gmail (Google account) username and password to unlock the handset, after several incorrect pattern entries have been recorded. Quite obviously this procedure has been put in place to prevent someone from brute forcing the unlock pattern. Unfortunately, the system is not as secure as it appears to be.
In some cases, possibly due to corruption of the stored data file, Android is known to reject the login information provided, even if it is correct. That’s what happened to me a short while back. Normally, the only way out would be to re-flash (reinstall the operating system) the device or to perform a factory reset. However, after doing a bit of digging I came across a surprisingly simple solution. The trick is to enter “null” (without quotes) as the password. In other words:
Userid = <enter your email id>
Password = null
The trick described above might not work for everyone. In that case you can try your luck with a firmware update/factory reset. Sony Ericsson users can download the Update Service application from here. If you have a HTC, Samsung or a Motorola handset, check out einartysen’s blog.
I am not sure if the problem is restricted to the Xperia X10 or not. Whatever be the case, the respective parties should fix this issue as soon as possible. Having “null” as password pretty much makes the entire security process redundant.