I'm a serious Eclipse fan, however I decided to quickly take a look at what's new in IntelliJ's IDEA 3.0. So, I go down the list:

New Code Refactorings. Interesting one that I've never seen before is "Replace Inheritance with Delegation", that's going to be extremely useful.  Also "Replace Constructor With Factory Method" since we all know that "new is considered harmful".

Intention Actions. Nothing new here, Eclipse's got 'em.

Code Inspection.  An audit tool however, suggests how to fix problems and fixes them in one click. Cool.

Enhanced JSP Support.  Much better than the alternative Eclipse JSP plugins.

XML Support.  Nice, Eclipse has similar options.

EJB Support. Hmmm, looks like you'll have to shell out a bundle to get this in Eclipse (see VSAD).

Method hierarchy and method call hierarchy browsers.  Method Call hierarchy, wow! 

Autogeneration of JavaDoc stubs. Done that seen that.

Code completion and highlighting in JavaDoc comments. Same as above, however does it integrate with XDoclet?

Local version control.  Finally, now they're learning something from Eclipse.

Compiler: dependency-based make and ability to specify multiple output paths.  Convenient.

Open API. Finally!

Multiple projects support.  Nothing new other than working of a new frame.

Code folding.  Seen it in JEdit however it'll be nice if Eclipse had this.

Splitting the current editor.  Impossible to do in Eclipse.

Storing a stack of the clipboard items.   Cool!

Navigation to a file by its name. Really convenient!

Project TODO's. Hope its better than Eclipse's version.

StarTeam integration.  I don't use it, prefer Perforce or CVS.

Quick and easy navigation from compilation errors to the corresponding source.  Don't know how useful this is.

New debugger features. Nice, Control of toString(). 

New in Jakarta Ant integration.  Cool, assign custom shortcuts. Code completion and highlighting, well grab a Eclipse plugin that does that.

Transparent Windows. Cool. 

Appearance of IDE is now fully customizable.  Well, that's leveraging Swing's look and feel.

New in JUnit integration. Nothing new to Eclipse.

Printing, Export to HTML.  That's convenient.

In summary, I'm surprised that the list kept going and going!  I use Eclipse everyday, however I've got to say, if you got the cash, IntelliJ 3.0 looks like a wise investment.  Given what I see in this feature list, looks like IntelliJ has the upper hand for now, and even after Eclipse 2.1 (which isn't out yet). 

One other observation, is that IDEA can be more innovative on the GUI side, that's because Swing is definitely more maleable than SWT.  SWT may be fast, however Swing is more agile, and just possibly with all these 2Ghz machines and up, it just won't matter.

So in conclusion, have cash and fast hardware get IDEA.  On a budget and have a budget PC, well stick to Eclipse.