Big success recovering deleted files
Let me tell you how I did it, just in case the information is useful to anyone out there who is in a similar situation with accidentally deleted files.
I recently had monitor trouble with my laptop (a huge purplish-black blotch that appeared at the top center of the screen and then spread out until it engulfed everything, made it pretty hard to do anything), so I borrowed a temporary laptop from work to use while my main one was getting repaired.
I copied all my personal files off to the temporary laptop while my main one was getting reparied.
Well, the main one is fine now, but when I was copying all those personal files back to my main laptop, I screwed up. I deleted my iPhoto library, thinking I had already copied it... but I hadn't. Oops! And like a dumbutt, I didn't notice this until after I had emptied the trash.
Good thing that deleted files are not actually gone. The 0s and 1s are all still there on the disk, it's just that the space is marked as free and the operating system doesn't know where the data is anymore. AS LONG AS YOU HAVE NOT SAVED ANY NEW FILES, which would overwrite the free space, the data is recoverable. You just need a program that grovels through the bytes on the unallocated disk blocks and pulls out any patterns that look like dead files.
I used a piece of command-line freeware called PhotoRec, which is available for all three platforms. It knows how to recognize all major picture formats — and plenty of other non-picture file formats as well — so you just check the file types you want, point it at a disk, tell it where to save recovered files, and let it run. I set it to find only JPGs, because that's what my iPhoto library was. (I don't know whether this was because iPhoto makes everything JPGs, or because my digital camera takes JPGs and iPhoto just copies them. Your mileage may vary.)
In my case, both computers were Macs, so I connected the target computer to my main laptop with a FireWire cable and started it up in "Target Mode", which you do by holding down the T key on the keyboard while starting it up. That makes it act as an external FireWire hard drive.
The only tricky part was telling PhotoRec which disk to search, because it displays a menu of the hardware disk names, which are not the same as the names that show up in the Finder ("Macintosh HD" etc). I figured out by using Disk Utilities that disk0 is my internal hard drive, and disk1 is the cd-rom drive, so the drive connected by FireWire was disk2. It shows up on the list twice, as "disk2" and "rdisk2". You want "rdisk2" because the "r" (for "raw") means a faster connection.
It was a 250GB drive, so the JPG hunt took many hours. I just left both laptops running overnight at work, and when I came back in the morning it was done and it had found 32,000+ JPGs. Many of these 32,000 are stuff I don't want, like UI cruft from GarageBand and those stupid party invitation templates that come with the Mac software. So I have some sorting and sifting to do in order to pull out the photos that I might actually want to use. But at least my photos are in there!