I have a solution for people who can't seem to get their phone recognised/detected in Windows, no matter how many combinations of USB ports/order of plugging in/opening program you try. This probably is caused by the drivers not installing properly, the drivers linked in the OP didn't work/install for me (Win7 64bit). Don't know if it's just my setup or because I have 64bit.
I found USB drivers for the Samsung Galaxy S that also work for the Omnia 7.
You can find them here (Get 64bit for 64bit Windows etc.): http://forum.xda-developers.com/showthread.php?t=728929
You'll get a folder of USB drivers after unzipping it, make note of its location.
This is what I did on my Windows 7 64bit PC (it may be different for you):
Make sure your phone is currently in "Download Mode" then plugged into your PC AND that WP7 Downloader is already open.
Go into Device Manager (Right click Computer -> Properties -> Device Manager, or search it in the Start Menu)
In the list find "Samsung CDMA technologies" or something similar to that. Note that it has a yellow warning icon for it.
If you can't find it, try changing USB ports. It might also say "Unknown Device" but only use this as a last option. If you STILL can't find it then your problem may be something else entirely.
Right click it and go to Update Driver
Choose "Browse my computer for driver software"
Browse to the folder of USB drivers you unzipped and make sure "include subfolders" is ticked
Go Next and it should install the correct driver
After this is done, 2 new
"Unknown devices" with yellow warning icons should appear in the list.
Do the same Update Driver method for these 2 devices as well. Note that you might already have an Unknown Device on your system. If this is the case then do the method for all of them (The non-phone ones should fail)
At this stage you should now have 3 new fully installed devices in the window:
"SAMSUNG USB Composite Device" (Under Universal Serial Bus controllers)
"SAMSUNG Mobile Modem" (Under Modems)
"SAMSUNG Mobile Modem Diagnostic Serial Port (WDM) (COM4)" (Under Ports)
That's it! It will now show up in WP7 Downloader, which should already be running. If doesn't show up, unplug the cable, wait a few seconds and then replug it into the SAME USB port. (The 3 devices listed above should still come up).
As I said this may not be the same with everyone and there's probably a shorter way of doing the above but it worked for me and I successfully flashed the Optus ROM. Would be good if OP could add this to the first post. (As a FAQ?)