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Relayed to me by a friend, I've verified that this works pretty nicely…

1. Visit the iTunes Music Store. Find a bunch of songs you like and drag them to an "iPhone" playlist. No need to purchase the songs - you're just adding the previews to the playlist.
2. Select the playlist, then export it ("File" -> "Export…") as a plain text file.
3. Open the text file in your text editor or even in Numbers (it's comma-delimited). Note that every preview file has a URL.
4. Download the 30-second previews (I like curl, but use whatever you'd like).
5. Sync the "ringtones" (née "previews") to your iPhone via iToner (this is the "almost" part).
6. There is no step 6!

I've done that with one song, and it works nicely. You won't get the pretty fades in and out, but the 30 seconds Apple chooses to preview is often pretty close to the 30 seconds you'd choose for a ringtone anyway, right?

As for me, I'll stick with buying the song, editing it in GarageBand or SoundTrack Pro (with faces), and sticking the resulting files on my iPhone. It's a lot more work, but I like the end result more.

Update: This method seems to have stopped working in either iTunes 7.4 or 7.4.1. Exported playlists no longer seem to contain track listings.