Version 3.1.0 includes a number of improvements over the 3.0 version:

- PreparedGeometry operations for very fast predicate testing.
  - Intersects()
  - Covers()
  - CoveredBy()
  - ContainsProperly()
- Easier builds under MSVC and OpenSolaris
- Thread-safe CAPI option
- IsValidReason added to CAPI
- CascadedUnion operation for fast unions of geometry sets
- Single-sided buffering operation added
- Numerous bug fixes.
  http://trac.osgeo.org/geos/query?status=closed&milestone=3.1.0&order=priority

(gdt)