Now this is just amazingly cool. iPhone hacker hachu developed a download plug-in for MobileSafari. Once installed and customized, it allows you to download data off the Internet and store it on your iPhone or iPod touch local disk in the /var/root/Downloads folder.
I downloaded the package (it's variously on RapidShare and BadOnGo; see the hackint0sh thread for links) and gave it a whirl. Following the instructions on the thread for modifying the Info.plist file, I told the plug-in to download MP3 files. (I had to disable playback as well in the QuickTime plug-in--so make sure to read the entire thread!) Now back in Safari, when I navigated to an MP3 file and selected it, this green Download button appeared. I tapped it, and the file downloaded perfectly. Before, the QuickTime plug-in would have just played the file back in Safari.
iPhone enthusiast RohitK asked if there was an easy way to play back these downloaded files using my software. As there wasn't at the time, I went ahead and modded my SendFile application to search ~/Downloads as well as ~/Media/Documents. You can grab a copy of the modded app here. This allows you to email, share and play back the downloaded media on demand, whether or not you are connected to the Internet.
Thanks to RohitK and Xadacka.