I had one of these phones with the exact same problem, brought it back to the shop and the staff member new about the fault and said that i could get a replacement brand new one. Seemingly Sony Ericsson are aware of the fault and are giving credit to the phone shops.
I love the phone so got another one due to the confidence of the sales person that I would not have any further issues and I haven't!!
The phone was three months old when they gave me a new one, pretty decent service I thought - it was an O2 Store.