I received a kindle2 back in January as a gift. I had assumed that it was a k2US and saw that it was on firmware 2.3 and had never prompted to update via WN (or been manually updated.) I was bummed that the 2.5.2 hadn't been posted for the k2us yet but thought I might update to 2.3.3 and maybe 2.3.4 manually to prepare to 2.5.2. I downloaded the k2us 2.3.3 but couldn't get it to work... when I checked my serial number I found that I had a k2i - I was pretty surprised!! Either way I was able to update to 2.3.3. > 2.3.4 and then 2.5.2. I actually found some links to the files elsewhere and since they were a bit faster and didn't require waiting between downloads - I used those. I did however verify using file hashes that they are exactly the same as the ones posted here. The links are:
http://www.filesavr.com/icfVgmNU 2.3.3
http://www.filesavr.com/3WOiALgc 2.3.4
http://www.filesavr.com/Wicx99kT 2.5.2
http://www.filesavr.com/5c5G1kgP All-in-one Zip
The zip file is nice since it's got all of the files in one download.