Well color me surprised - after trying a few convoluted methods for exporting some of my NetNewsWire feeds in a format that Safari could import (as RSS, not HTML links to real sites), I finally realized that either Brent Simmons and/or Apple have already done some of the heavy lifting. If you toggle Safari's Show All Bookmarks view from the Bookmarks menu (or simply use ??????B), you can simply drag and drop one or multiple selected feeds from NetNewsWire into Safari's bookmark editing window. Unfortunately, you can't drop a Group of feeds from NetNewsWire, but this trick should satisfy most users who need to get some feeds into Apple's browser.
When you import feeds into Safari like this, it won't recognize them as true RSS feeds (or begin downloading headlines and displaying unread counts) until you visit them at least once. After that you're all ready to roll with a few (or all?) of your favorite NetNewsWire feeds in Safari.
If you use a desktop-based RSS reader other than NetNewsWire, be sure to let us know if this same trick works for your reader.