Modifying UI in Star Wars: The Old Republic

This is an old question but the current answer is now out of date.

As of Patch 1.2 the game supports the ability to move around most or possibly all of the ui elements that appear on screen and in general has a lot more customisation options than at launch.

http://www.swtor.com/info/media/trailers/quick-look-ui-customization is an official video demonstrating this.

You can access this through a small button just above the lock toolbars button on your toolbar or through the main options menu (ie where you'd go to select change preferences).

It includes three different defaults that you can use either as is or as a starting point ofr your own customisations.

The three options are a simple form with one action bar, the extended quickbars which gives you all the quickbars active and one that seems very similar to the standard WoW layout that I think is called "Classic" or something similar.

If you're thinking of UI customization a la WoW, the short answer is not really. There are no addons or LUA scripting in SWTOR that would allow you to fully customize the UI.

However, Chris' answer is correct in that you can certainly perform some modifications to your UI, such as resizing elements and placing them on different portions of your screen.