NOTE: This list is always a bit out of date. Your best bet is to monitor the "layout" newsgroup and mailing list - visit the community page for info on how to get access. Todd Fahrner of the Web Standards Group has published a document that sums up a few recent posts to the layout newsgroup outlining what needs to be done and how you can help.
Macintosh and Unix porting. The NGLayout team is (currently, unfortunately, sadly, regrettably) under staffed in the Mac and Unix departments. We could really use your help here! Send mail to kostello if you want to help.
XML parsing, XML processing, XSL transformations - send mail to rickg if you want to help.
Wrap up the RDF BE with XPCOM interfaces - send mail to jgellman if you want to help.
Integrating NGLayout into the public source browser. The integration team at Netscape is already hard at work on this issue. If you would like to help, send mail to jevering. Step one is to modularize the mozilla codebase.

If you, your team, or someone you know is interested in this opportunity, please send email to Rick Gessner.