How to Check if your Sony Ericsson Xperia X10, X10i or X8 are Permanently Locked / Hard Locked?
Type the following sequence from the lock menu Sony Ericsson X10, X10i, X10 Mini, X10 Mini Pro, or X8 device
Menu => Back => Back => Menu => Back => Menu => Menu =>Back OR dial: *#*#7378423#*#* 
This works on my x10 running Android 2.3.3
Select Service Menu and SIM Lock
You will see a new menu that looks like this:

[x] network 10
[] network subset 10
[] service provider 10
[] cororate 10
[] sim 0

If there is an X beside any of the categories, then means you need specific code to unlock that phone.  
Normally, Sony Ericsson phones come only with a network lock as active.  
The most important part of this is to view the numbers beside each category. 
That number represents the number of tries you have left before your Sony Ericsson device becomes permanently hard ... Read more »