It's happened twice actually, and I'll provide them in reverse order since the first was more exciting.
The second time I was with an instrument student in his Mooney slightly north of Albany, OR
on V23 at 4000 (I think, could be wrong on the altitude), and a Citation shot by just underneath us climbing towards our 2 o'clock. It really wasn't all that close, probably 300 feet or so below us, but he quickly climbed above us after passing. Center called the traffic for us just after
it passed (though Center still saw it at our 7 o'clock about) and I can't remember exactly what I replied with but it was something about him being close.
The first time I was in a Cutlass over the Columbia southeast of Scappoose, OR
at about 1,500 I think. Saw something move on the edge of my periphial vision on the right, look over, and there's a twin (something like a Baron) in a 90 degree bank pulling away from me. He (she?) didn't have any bugs on his belly, but there were a lot of grease stains. If he'd (or she'd) reacted a fraction of a second later, we'd both be dead.
Both situations illustrate that scanning for traffic doesn't always work, since I was in both cases.
[Edited 2003-12-02 22:32:02]