I'm interested to control Arduino micro controller using Mach3. Has anybody got any experience or samples how to do. I would like to do things like set and read spindle speed and limit switches.

Arduino is very easy to program microcontroller and it can communicate with PC using RS232 over USB. More information from http://arduino.cc/

I assume you are then using the microcontroller to control some higher function in a robot or something similar, otherwise I cannot see in what way you are expecting Mach to "control" the Arduino.

Mach puts out onlt step/direction pulses for axis and then can put out various on/off signals to control relays etc. As you say it can read on/off signals into the computer as well.

The control of spindle speedhas two different systems, step/dir (as in the axis control) and PWM - pulse wave modulation - which requires additional electronics.

If you use a breakout board on the LPT1 port, you can read the output signals directly (and drive TTL - and therefore a micro controller directly) with recourse to RS232.

Mach does use a USB output to drive the latest "smooth stepper" control card, but I do now know what protocol is used for the link.