The link Phalanx7621 posted was supposed to point to the Download Repository (link is in my signature.)
The preferred version of Odin is 1.85. If you need it, the download link for Odin3 v1.85 at the top of the Download Repository is a working link.
If the boot loader was corrupt, the phone would be a true brick (no sign of life.) If the secondary boot loader was corrupt, the phone would not boot as far as the Samsung logo. It sounds to me like the AT&T Samsung specialist you received that information from was not fully informed.
The I9100 splash screen with the yellow warning triangle is due to flashing a custom kernel, so nothing to worry about. It can be cleared by flashing a stock kernel. Using the following procedure would have put that on your phone anyway by flashing the Siyah kernel, and then will removed it when stock Gingerbread is flashed.
Please note: The link in the Download Repository for the stock Gingerbread that you need to flash is not working, and I have yet to re-upload all these items to a new host. You will have to download this from the sammobile firmware page. It is the I777UCKH7 / I777ATTKH7 dated 2011 August. You'll need to sign in to download it. The registration is free. Edit: sammobile has not uploaded this file to a new host yet either. I will have to upload it and the sbl for you, but since today is Christmas, you may have to wait a day or two. At least you know that there will eventually be something to try.
Edit: The required files are now available with Dev-Host links in the Download Repository.
I think you will probably recover the phone by assuming it has nand read/write corruption and following this sequence:
Instructions to clear nand read/write corruption.
Odin3 v1.85 is recommended.
1) Download the Tar version of Siyah 2.6.14 Kernel
. Flash it in PDA. Without ticking Re-Partitioning
2) Flash the Tar.md5 of the full stock Gingerbread distribution from the Download Repository, I777UCKH7 including the bootloaders and everything, in the PDA slot.
3) If that is successful you are done. If it is not, then:
4) Download the SBL Bootloader from the Download Repository. Flash it as PDA in ODIN, without ticking Re-Partitioning. (Please observe normal bootloader flashing caution.)
5) Flash the Kernel, as in step 1.
6) Flash the full stock Gingerbread distribution, as in step 2.