The default format for running the High Voltage SID Collection (HVSC) on the device will be in a ZIP archive. The HVSC is about 65mb zip'ed up, which in turn contains a 200mb uncompressed archive, C64Music.zip. This inner archive contains over 35,000 files. It is impractical to expect that will extract on the device in a reasonable amount of time, hence the decision to stick with an archive. I will provide a re-compressed version of C64Music.zip, which you will be able to download via mSID and from then on, one should be able to download the HVSC updates direct from their site. I will also include a sqlite database of all the files, in order to improve performance for searching. I expect that long-term, the role of the database will grow to become your store of ratings, play lists, etc. Given it's sqlite, it will be open to reading and updating.
From an implementation perspective, I have pulled together the necessary code to be able to read a ZIP archive as Cocoa does not have ZIP support built in. I have built this as a .framework, and will probably open-source it at some point.
The browser will use the familiar hierarchical navigation built in to the iPhone.