Not much is known about the fixes and improvements but according to reports, the new system refresh version brings performance and battery life improvements. Also, the phone is reportedly switching faster between 2G and 3G networks and touch sensitivity seems to be improved, with multi-touch being enabled and working. There are also some improvements in the browser as Internet appears to be working faster according to user reports.
Source: XDA Developers, PTCRB
Via: Xperia X10 Blog