[G]atesphere, one of our active forum members, tipped us about his PIR sensor interface to the MSP430 Launchpad.  He ran into some problems in getting the Launchpad to detect pulses from the PIR sensor. After some debugging and help from his forum thread, it was found that the output of the PIR was too low for the MSP430 to register it as a high signal.  He says:

The LaunchPad operates at 3.3V, whereas the Arduino (or, at least, the Arduino Duemilanove) operates at 5V. When operating from a 5V source, the output pin pulses at ~3V. But when operating from a 3.3V source, such as the LaunchPad, the pulse is a mere ~1.7V. The MSP430 line, as juani_c from the 43oh! Forums points out, requires 0.7*Vcc, or ~2.31V at Vcc=3.3V, to register as HIGH. (It seems the ATMEL uC’s are much more sensitive) So clearly, powering from the LaunchPad’s standard 3.3V is not going to be a valid option.

He solves it by hooking the PIR to the Launchpad’s 5V USB tie points instead of the 3.3Vcc.  Code is available on his blog page. Thanks [G]atesphere!