Prior to Wordpress 2.1, the wp_list_cats() function has a “children” parameter to hide your subcategories. For whatever reason, wp_list_cats() was deprecated, and the “children” parameter was dropped from the new wp_list_categories() function in WP2.1 and onwards.
So how to hide the unsightly subcategories from showing up in your sidebar? You can install and use the new function in the unfold_list_categories plugin by Braydon Fuller.
Or hide it with a quick and dirty trick using CSS. Simply open your style.css file and add the following code. And poof it’s gone 
.children { display:none; }
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You rock- seriously. Entire (counter-intuitive) plugins have been written to do what you just did with a simple stroke of common sense.
Thanks for this!
Just because of the google search results…
depth=1 now works from 2.5+
From docs…
depth
(integer) This parameter controls how many levels in the hierarchy of Categories are to be included in the list of Categories. The default value is 0 (display all Categories and their children). This parameter added at Version 2.5
0 – All Categories and child Categories (Default).
-1 – All Categories displayed in flat (no indent) form (overrides hierarchical).
1 – Show only top level Categories
n – Value of n (some number) specifies the depth (or level) to descend in displaying Categories
Thought it might be of some use to someone.