You can also test the "corruption" of an ipk by trying to read/open it directly with tar.
tar tzf package.ipk
Should return 3 files
./control.tar.gz
./data.tar.gz
./debian-binary
If that fails then most probably have picked up a corruption during download OR you have a ar compressed ipk. use "file" to find out what type of ipk you have, for Sharp compatible ipk you should get "gzip compressed data" or something indicating it has been gzipped. If it says something about "debian" or "ar compressed" then you have the wrong kind of ipk and will need to convert it.
The other option (and one that happens a lot when I'm forced to use IE) is that the browser gunzipped the ipk for you during download (this will show as "tar" with "file")
Stu