RARLabs already provides an official version of WinRAR CE for Windows Mobile 5.0 devices but not for Nokia S60 or Sony Ericsson UIQ.
MobileRAR 0.8 is a freeware J2ME MIDP based RAR files unpacker/extractor for Java capable mobile devices including Nokia S60 3rd Edition devices like Nokia N80, Nokia E-Series, Nokia N93, etc. I haven’t tested this on Sony Ericsson but being a Java capable phone it should install fine.
The website and application are in Russian but there’s no better freeware RAR unpacker for mobile devices.
Please do let me know if it installs on Sony Ericsson so that I can update the post.

Download Link- http://mobilerar.narod.ru/MobileRar.zip