Bukkit

• Borders can now have a separate X and Z radius specified, so that rectangular and elliptic border shapes are supported. The set, radius, shape, and wshape commands have been updated to support this. Performance shouldn't be affected by this feature. Thanks to IMathe172I (Lang Lukas) for contributing the code for this!
• New command /wb [world] setcorners <x1> <z1> <x2> <z2> which will set a border based on corner coordinates from location (x1, z1) to (x2, z2). It's essentially an alternate to the standard set command and requires the same worldborder.set permission to use.
• Players who are flying (from creative mode or otherwise) can now be moved back into air when they cross the border and are knocked back; no more requirement for a safe place to stand for them.
• Workaround fix for players in vehicles (boats, minecarts, pigs, whatever) being able to go beyond the border. This fix was necessary since the Bukkit devs recently made teleportation of entities with passengers simply fail without even throwing an error (argh), apparently as a workaround of their own for a Minecraft bug. Such players knocked back in a vehicle will now likely see themselves out of the vehicle briefly due to the clunky workaround I needed to implement.
• Players going through a portal will now no longer be able to generate chunks outside the border; the portal teleport location is re-targeted inside the border to prevent that so that new portals aren't created outside the border.
• Changed method for finding region folder (used by Fill and Trim) to hopefully support custom world types such as Twilight Forest and Mystcraft... untested, but hopefully working.
• Added MC 1.5 redstone comparator and activator rail to "safe open blocks" list for the knockback safe location finder.
• Minor change to the Fill command to make it potentially less taxing when run at high rates.

New commands:
/wb [world] setcorners <x1> <z1> <x2> <z2> - set a border based on corner coordinates from location (x1, z1) to (x2, z2).
/wb shape <elliptic|rectangular> - exactly the same as /wb shape <round|square>, with elliptic=round and rectangular=square; both command variants left in for backwards compatibility, but they do the exact same thing.
/wb wshape [world] <elliptic|rectangular|default> - exactly the same as /wb wshape [world] <round|square|default>, with elliptic=round and rectangular=square; both command variants left in for backwards compatibility, but they do the exact same thing.

Updated commands:
/wb set <radiusX> [radiusZ] - now has optional radiusZ value, to specify X and Z radius separately; if radiusZ isn't specified, X and Z radius both use the same specified value for radiusX
/wb [world] set <radiusX> [radiusZ] <x> <z> - as above: new but optional radiusZ value is added
/wb [world] radius <radiusX> [radiusZ] - as above: new but optional radiusZ value is added

Elliptic border shape example, radius 400x150: