I have been developing a Seam application in IJ 6 for almost a year now. It has worked simply. The only real extra support that could be added would be for SeamGen. All the other parts are really already handled by IJ, such as JSF, EJB3, and any AS.
Also, if there was any extra to be built, it would most likely be a third party Plugin, which anyone can build. Unfortunately, JBoss is mostly concerned with creating their "JBoss IDE" which are plugins for Eclipse.
In fact IntelliJ IDEA understands Seam-using code quite well, and provides quite a lot of smart highlighting and navigation features, although there is no big "Seam support" area in the UI. The only caveat is that you'll need to map the .xhtml extension to the JSP file type.
If you would like to see some additional support, feature requests for this area are always very much welcome.