Ahead of schedule, Ultrasphinx 1.9 is out with delta indexing, ERB support in the
.base files, and official compatibility with Sphinx 0.9.8-rc1.
what it is
Delta indexing speeds up your updates by not reindexing the entire dataset every time. Instead, it keeps a main index which is updated rarely, and a delta index, which is updated frequently and only contains recently changed records.
Of course, your records need timestamps for this to work.
Note that there are some gotchas surrounding Sphinx and index merges, mainly that facet counts and text sorting may not be perfectly accurate. In an append-rich environment (most web apps) these tend not to matter.