I highly recommend staying as far away from the current OpenACS 4 portal package as possible (it's a direct port of the ACS 4 classic portal package).
The new one written by Open Force is much better - I'm using it on a dotLRN package I'm writing. It's not been released yet but Open Force intends to in a matter of weeks.
It allows you to use templates for portlets rather than straight Tcl, supports theming and multiple styles, etc. It's looking good and is close to being finished.
