I just had the same problem, tried to flash a bad ROM and thought I was done for. The information on these forums is a bit scattered so I will just detail what I did to unbrick it.
1. First thing you need to do is have the samsung drivers, if you're on Windows 7 or above, Windows can probably go find them for you when it comes to plugging your phone in.
2. You need to have the original stock firmware, luckily they are essentially all the same so here is the Open Australia version I used (I'm on O2 in the UK) http://www.mediafire.com/?903oq9l7qbmkir4
(password is: mod2xtreme.com)
3: Extract that folder somewhere you will remember.
4: Odin is a firmware flasher, it's what will save your phone. It needs a PIT file, luckily some kind person has already packaged the PIT for the Ace within Odin here: http://forum.xda-developers.com/atta...9&d=1345402010
Now you have all the different pieces required
5: Open Odin on your computer and put your phone into Odin mode by holding the power button, volume up and volume down. (and read the screen, it'll ask if you if you're sure and to press volume up) - once you plug the phone in correctly and the drivers are installed Odin should come up saying "<ID> Added!!"
6: A picture paints 1000 words here: http://i.imgur.com/La1r4zE.png
- you should ensure you select your PIT file, and all the correct files from step 3 (modem = phone) - click start then wait for it to say "Pass" (I had a nervous smoke here)
7: Boot your phone into recovery by pressing the home button, volume up and power. Let it run through the update and wipe the cache and reboot itself.
8: You're in! It went German on me so I had to change the language at the startup screen, but other than that it's fine.