I have read here about the possibility of Multi-signature transactions. Is there a statistic available how much of these are done?
Blockchain doesn't currently offer that and the only way to construct a transaction like that is to build it manually. So as a result both the sender and each recipient must have l33t skills.
This capability will come, it is just that since that was introduced, scalability had to take priority over new features for most Bitcoin development.
They happen but rarely. Here's an example of Bitcoin address used recently (March 5, 2013) in an escrow transaction:
Here is nice statistics http://webbtc.com/stats of different script type
I wrote some analysis of the multisig P2SH transactions in the blockchain; http://www.coindesk.com/year-multisig-so-far/ Around April this year there were like 7,000 transactions. I have the frequency per 2500 blocks, showing a nice histogram of their increasing usage, and also the top 4 address types (2-of-3, 2-of-2, etc).