I actually disagree with OTD nailing it on the head. Here's my case.
I discovered this a while ago while installing the original Swype (or not).
When I download the .apk file from my Windows XP Pro web browser, dragged it onto my SD card of my phone, and tried to install, I'd get that there was a problem trying to Parse the installer package.
Okay. So I browse to the site with my phone's web browser, downloaded the file directly with my phone, and opened it up. No problem installing.
What?
I drag the freshly downloaded file from my phone to my computer. I check the md5. EXACTLY THE SAME.
Baffling? I can repeat this experiment again and again and again. Different web browsers. Different SD cards. Same result.
Something is different when you use Windows to download it, it seems. I have 100% success with .APK files and other things on my phone, but strange problems when using any version of things from the computer.
As a computer technician, I have NO good explanation for this. None, whatsoever. Files, same size, same hash. One works. One fails to parse. I have to imagine that something along the same lines might be possible with the 2.1 image.
I downloaded it straight to the phone for my gf's Eris, and had little problem (other than the known 'bugs' like Facebook Syncing). Perhaps others with more vast problems have downloaded it on the computer, then dragged it to the SD card?