A portal which is JSR168 compliant is guaranteed to correctly manage and present a JSR168 compliant portlet. In essence its design follows all the mandatory requirements of the specification.
Offering JSR168 "support" offers no such guarantees, but its always possible for people to phrase themselves badly. If you need the details you should check the documentation and consult with the vendor.
Generally speaking you will find that an open source portal that is based upon the Pluto container will be JSR168 compliant since Pluto is the reference implementation.
For details of what's actually mandated in the specification, you can download the public review copy from the JCP website: http://www.jcp.org/aboutJava/communityprocess/review/jsr168/
I can also recommend a really good book on the subject :-)