I'm using Windows XP.  The folder paths should be the same or really similar on Vista but I just don't know that for sure.  There are some UT3 cache converters out there but the ones I have tried do NOT do every step outlined here.

1, Go to [color=yellow:cb889e224b]My Documents\My Games\Unreal Tournament 3\UTGame\Cache[/color:cb889e224b] and open the cache.ini file with a text editor like Notepad.

2, Find the line in cache.ini that references a map you have downloaded.  In this video I'm using VCTF-TheMoonRulz as the example.  All maps should have the gametype abbreviation at the beginning of their name - VCTF in this example.

3, The string of characters before the = is the name of the file in your cache folder.  The string after the = is the proper name for the file.  UT3 map files need to have a .ut3 file extension.  Find the file in your cache folder that matches the string before the =.

4, Copy the proper name of the map file and rename the file you matched up.  Don't forget the [color=yellow:cb889e224b][size=18:cb889e224b].ut3[/size:cb889e224b][/color:cb889e224b] file extension.

5, Cut the renamed file from your cache folder and paste it into your CustomMaps folder.
[color=yellow:cb889e224b]My Documents\My Games\Unreal Tournament 3\UTGame\Published\CookedPC\CustomMaps[/color:cb889e224b]

6, Remove the line from your cache.ini that refers to the file you moved out of your cache folder and save the cache.ini file.

7, To see if it worked, start UT3, go to Instant Action and see if you can play the map offline in single player mode.

This version is just for people like matt who can't read:
<embed src="http://www.youtube.com/v/ZQQTWtrbkdA&amp;ap=%2526fmt%3D18&amp;hl=en&amp;fs=1&amp;color1=0x272727&amp;color2=0x94000E&amp;border=1" type="application/x-shockwave-flash" allowfullscreen="true" width="640" height="480"></embed>

If you don't like my instructions or video, I suggest you try [url=http://www.google.com/search?num=100&amp;hl=en&amp;newwindow=1&amp;safe=off&amp;q=ut3+cache&amp;btnG=Search]GOOGLE[/url].
The v2.1 of BradG's cache cleaner actually seems to be working properly in that it will delete files like it's supposed to.  BradG's site is gone AFAIK but there's still a download here:
http://files.filefront.com/UT3+Cache+Converter+V21/;10142580;/fileinfo.html