Can somebody help me with step-by-step list of how to create a multi signature 2-of-3 transaction?
Actually, both receiving funds at and spending funds from a multi-sig address is already supported.
You can find the code to do so, plus comments, by Gavin:
It seems some partial implementation exists. Quoting Gavin:
Generate a multisig address:
addmultisigaddress returns the multisignature address. Be a little careful, the public keys are raw hexadecimal and don't contain checksums like bitcoin addresses do. You can then send funds into that 2-of-3 transaction using the normal sendtoaddress/sendmany RPC commands, or the GUI (or anything that's been updated to recognize multisig addresses).
Spend from a multisig address:
This isn't implemented yet (RPC calls mechanism). You could gather up all addresses into one wallet, and then you would be able to spend the funds.
Update March 2014
Multisig is still hard. I believe the SX library is an easy tool (by people like Vitalik Buterin and Amir Taaki) for working with multisig, but I haven't tested that myself.