Introduction
You can write your own firmware to make the device do something completely different. Use any compiler tool that supports the PIC16F873.
Kent wrote a handy tutorial on getting started with PIC programming here.
Note: Use these instructions to flash the device.
CCS C Compiler
You can find a copy of the CCS compiler in /net/hm41/folks/will/btacc/ccsc.zip. This is all you need to generate .hex files to be downloaded to the device. See the file Ccsc.chm for language reference (windows help file). It comes from a newer version of CCSC, so some functions are not available. Use ccsc.bat to compile or make sure to ' SET PCINCLUDE=d:\pic\PICC\EXAMPLES' correctly.
If you're going to flash the firmware via Bluetooth using the bootloader, but sure to include #org 0x0F00, 0x0FFF void loader16F873(void) {} in your source code to accommodate the bootloader.
Code
Here is the current firmware for accelerometer sampling: adc5.c
And the corresponding .hex file: adc5.HEX (compiled for use with bootloader)
Flashing
When you're ready to flash the device, follow these instructions.