The MIT High-Low Tech Lab posted a tutorial on how to program an ATtiny45 or ATtiny85 micro-controller using the Arduino IDE.  This functionality is accomplished by using a subset of the Arduino libraries to add a set of core runtime source files to support the ATtiny45/85.  Matt Richardson of MAKE Magazine posted a video demonstrating how to use the Arduino IDE to program the ATtiny.

There is also an open source project called the “Arduino-Tiny” that enables Arduino users to work with the ATtiny84/44/24, ATtiny85/45/25, and ATtiny2313.

** Update ** This tutorial has been updated for the Arduino IDE V1.0 and includes some updated commands.  Check it out here.