What are the limitations of intellectual property between myself and my employer?

I work for a large corporation as a software engineer. My work entails a number of things one piece of which is a web application which serves data using a number of new Web 2.0 technologies. If I write a web application which is complete and totally unrelated to my work, but never the less uses the same web 2.0 technologies does my employer own my work?

To give a comparable example the work I do could be like a control system for a robotics plant, and the web app displays status of the robotics, and the application I wrote on my own time is an online word processor.
Any case law examples would be tottally tublar
Thanks clear some things up with my coworkers

You probably signed an agreement when you were hired that gave all the rights to anything you develop to the company. Check your personnel file. If not, then there may be case law that would give the rights to them as you had access to the technology at work even if your application is not related to the work you do.

If you work for a large employer you certainly signed several agreements the first day, an NDA, a non-compete, and a IP assignment form (they may have been combined).

Your HR rep can get a copy of that for you to remind you.

Even without an agreement, all work you do for the emplyer is work for hire.

If you use any of their IP, including processes on your own outside of work, it becomes fuzzier, but they could make things unpleasant for you and you might find you don't own the work.

As for a defense, prior art (menacing you (could have) learned about it outside your work, is a very good one. In the case of a word processor, there are tons of prior art in texts and actual source code.

In fact, you may be more likely to be violating the IP of one of those sources (especially anything GPL'd) then anything from work.