Short version: Samsung Kies (Samsung's own program for backing up and upgrading Samsung phones) bricked my Galaxy S while upgrading it. Combining Heimdall with some recipes from other blogs allowed me to save the device.
Long version: Kies (which seems to cause a terrible amount of problems for its users) was upgrading my phone to Android 2.2 firmware when it decided to give up and left the phone more or less dead.
On power-on the phone showed a funny little icon. Not sure what it was meant to show. Two small things connected somehow.
Pressing volume-down and Home while powering on got me a big, scary
digging android icon. At this stage, Kies no longer detected the phone (and lsusb showed it as modem, not a phone).
Googling revealed that almost a googol users have had their phones wrecked by Kies, and quite a few helpful posts.
Anguel Nolastname ran into a similar problem, but I managed without the custom USB cable he had made.
Using Heimdall with Richthofen's files, I was able to flash the firmware:
tar xf CODE_I9000XFJP7.tar.md5
tar xf CSC_I9000DBTJP3.tar.md5
tar xf MODEM_I9000XXJPP.tar.md5
heimdall flash -verbose --factoryfs factoryfs.rfs --cache cache.rfs --dbdata dbdata.rfc --kernel zImage --modem modem.bin
The result booted. WLAN doesn't work, which seems to be a common problem after Galaxy upgrades. A factory reset may help, but right now I have some Christmas presents to wrap and no time for more upgrade troubles. WLAN can wait.
Richthofen suggests using Odin to flash, but I think Heimdall does better, since Odin requires a two-step process and Heimdall flashes everything at once (I think that's why Anguel needed a magic cable and I didn't).
Some data was lost, including the contacts. Kies has a backup copy, which it won't restore. What a fine program.