Yes it is.

It is just a different flavor of Symbian. Nokia uses the S60 version of Symbian, and SonyEricsson uses UIQ. They both develop it themselves but the core is the same between them.

The biggest problem with SonyEricsson is that almost none of their phones have EDGE, so unless you get one that supports 3G in the US, you are screwed and stuck on GPRS, also when out of the 3G coverage area you drop back to GPRS.

FWIW, even the specs page at SonyEricsson says it's Symbian...