Get it from the download page: http://www.arduino.cc/en/Main/Software
Please note that all Mac OS X users will need to run the new macosx_setup.command file included in the distribution (and possibly reboot).
Also, you'll get some error messages the first time you run the software, just ignore them - they're warnings from compilation of the libraries.
Changes include: Main sketch is now compiled as C++ (instead of C). Updated avr toolchain. printInteger(), printHex(), etc. now handle longs. millis() fixed (now overflows after days, not minutes) Fixed path to java in Windows run.bat. Added Matrix and Sprite libraries (written with Nicholas Zambetti). PWM now working on pin 11 (in addition to pins 9 and 10). Slowed PWM frequency (on all three PWM pins) to 1KHz. Now give an error if compiled sketch is too big. Fixed abs(), min(), max(), and constrain() macros. Added menu items to the IDE to burn bootloader. Now display binary sketch size on upload, and give error if too big. Added C++ serial library. Resynced with Processing/Wiring IDE code (improved auto-format, faster logging to serial monitor console, other bug fixes) New library system (gives a few spurious errors when Arduino starts). Updated to latest version of the RXTX serial library; Mac users will need to rerun macosx_setup.command.