First, what's the tag cache? s3mp is, at least for now, almost entirely client-side, so to get the meta data (artist, song title) of an mp3, it downloads a small initial chunk and a smaller final chunk from the file, looks for tags, and if it finds them, stores them. The process of looking takes a while, but if all the tags are already stored in a handful of files, the process is a lot faster.
I changed how tag caching is handled. Instead of letting the user choose a bucket to store tags in, s3mp now (once this beta is released) automatically stores a tag cache (with intentionally obvious names like .s3mp_tag_cache.0.xml.z) for the all mp3s in a given bucket in that same bucket. I had an ugly dilemma to sort through on this one. A tag cache for each bucket is easy for both me and the user, and loading a single bucket, or making it public to share, works as expected. The problem is in how it's handled when the user doesn't have permission to write in that bucket. All of the solutions put a strange burden on the user to pick, with foresight, where to put the tags.
For now, if you want to use the tag cache, make sure you have write access on the bucket.
The Progress Slider is making a lot of, um, progress. The UI basically works, it just needs keyboard support, and the progress portion already indicates progress downloading a song.