I have been asked by a partially sighted friend of mine to investigate a way of producing a tilt sensor that could be put on a tray.
I have been looking at using an ADXL245 accelerometer with instructions from Sparkfun here using I2C and looking at the attiny datasheet I can see that the pins 5 and 7 are SDA and SCL.
Before I go ahead and buy all the parts and download the libraries etc would someone please confirm for me that the software would be compatible with the attiny? I know Serial.monitor is not available on the attiny but I could do all the testing on an uno and move to the attiny when I am happy with it.
Would the software use the pins 5 and 7 on the attiny automatically or would it be simple enough to change the pins?
With ATtinys you can use software serial for the serial monitor and software I2C for the sensor (also called "bit bang" serial). One some ATtinys there is a built in USI serial interface which can be used for either.
Which ATtiny and why go to the bother, when Arduino Pro Mini knockoffs are $2 on eBay?
Before I go ahead and buy all the parts and download the libraries etc would someone please confirm for me that the software would be compatible with the attiny? I know Serial.monitor is not available on the attiny but I could do all the testing on an uno and move to the attiny when I am happy with it.
That depends on the IDE core you use. There are several ATtiny cores available, some include a Wire library which has the same interface as the standard Arduino library, other depend on TinyWire{S,M} library for the same purpose. The TinyWire* libraries are not interface compatible to the Wire library so there you must change the ADXL library to make it work.
Would the software use the pins 5 and 7 on the attiny automatically or would it be simple enough to change the pins?
We cannot answer this question as you didn't tell us which Tiny you're planning to use. In general you cannot change the pins as the hardware is wired to specific pins.