It supports JSTL, because that's just a tag library. IDEA 3.0 has full support for tag libraries.
If you are looking for support for Expression Language (EL), which is commonly used with JSTL, you will have to wait for 3.1 or maybe much later - it's currently an open Feature Request, and not planned for Aradnia (which is the beta for 3.0)
It supports JSTL, because that's just a tag library. IDEA 3.0 has full support for tag libraries.
If you are looking for support for Expression Language (EL), which is commonly used with JSTL, you will have to wait for 3.1 or maybe much later - it's currently an open Feature Request, and not planned for Aradnia (which is the beta for 3.0)
Refer to the feature request here: http://www.intellij.net/tracker/idea/viewSCR?publicId=5124