Dave Winer is going to release an Out Lining app that uses OPML natively (and runs on Ms Windows and Mac Os X). He intends to call it OPML, not having come up with a good/available alternative.
How about 'Op Liner'? Hmm, 'opline' gets used in computer terms, though I don't think there's a piece of software with that name...
How about 'XLiner'?
Hmm, I wonder whether Dave has ever tried Buzz Outliner? It edits OPML, too... Oy, I wonder whether they'll handle each other's file nicely.
Jun'2005 update: it looks like this is being tied into Instant Outlining.