Hey everyone!
I am a believer in OpenSource/Wiki ideals so when I saw that there’s a mapping project of that sort I got interested. I have access to a car, a GPSr, and even an iPhone (and I picked up one of the OSM apps for it) so I’d love to get to work 
How do I tell what areas need to be mapped? I wouldn’t mind driving around to track out some roads but I want to make sure that I am not doing an area that someone has already done (unless it needs to be done again.) Is there a way to find what sections have been modified as opposed to the sections that are still just the basic Tiger map? Also, is there any sort of coordinated effort going on in the Northwest? I’m looking mainly in Southern Washington and Northern Oregon. Any other helpful hints you might have for a newbie would also be appreciated
One of the approaches I have used to find new areas is edit the map and scan around over the yahoo map to see areas where there is fresh dirt (roads under construction) with no road ways. I jot the neighborhoods down and make a point to visit the area on my next trip. What would really be nice is a printout of different areas from OSM which you could mark with a highlighter. I don’t know a way to make nice printouts (there is always PRINT SCREEN but nothing like convert to PDF that I know of, anyone???)
You can also use the NoName map to find roads that need names to be filled in, which you can find by visiting the area and jotting down or getting your hands on the state/county GIS data.
Another option check out this free app http://itoworld.blogspot.com/2008/07/new-osm-mapper-for-openstreetmap.html which allows you to see the activity and edits going on in your area. Seems to be downloaded nightly. You can see the popular users and try to contact them.
How about focusing on just the trails. Get a trail map from the city/county and go exploring. You might also find some new trails - happened to me, a couple new greenways popped up I had no clue about. When I scanned Portland, Forest park trails look non-existent.
Some approaches - hitting the entire area with just one type of data - say, all the footpaths. Or, focus on a single park and completely map it out - all the roads, parking, toilets, ammenities, etc. Or, go thru the existing data for one type and clean it up - straighten all the railways to match the aerial photos, or add in the power lines and power stations, etc.
Having lived in Portland for 30 years, my first inclination would be to map the brewpubs 
Have fun!
You might also try this method if you are savvy with computers and GIS tools