You need download mode to be able to use odin. Odin basically does a clean flash of the phone from within this mode and allows for pretty much any problem which can arise to be solved.
If you're going to do something which invalidates the warranty, its a very good idea to ensure this is working.
Would i be better off taking my phone into t-mobile then and telling them to sort it? Im wanting to update my phone to froyo when t-mobile finally release it.
Thanks