I'm surprised a mod hasn't moved this thread to the themes forum.
tgoulart, I'm not up on what Cingular does to the OS..but you might look at this thread and see if it's applicable to you.
BlackBerryForums.com : Your Number One BlackBerry Community - View Single Post - OS 4.1.0.284 but no Zen theme
The gist of it is this. By removing the vendor id (each provider has a number assigned by RIM), the Desktop Software recognizes that there is now an OS component that can be installed and it uploads it.
CAUTION: Read and understand the link first before moving on. ALSO, understand this is for T-Mobile and may not be applicable to Cingular. I simply provide it as is since no more knowledgeable Cingular people have wandered in here yet.
Luck.