comments and privacy in the bitcoin transactions and blockchain records.

Hi all,
I am still quite new to bitcoin so I´d like to just say hi to all before I ask my first question.

I understand the concept of privacy in the block-chain, that transactions are simply tied to addresses and there can be a virtually infinite amount of those so peoples identities in relation to those addresses is basically disconnected.

But I am also aware that bitcoin is actually perfectly transparent when it comes to the transactions and the wallets, searchable all the way back to the very first transactions. All saved and shared and passed on , update and verified between each mining node.
I would imagine that the minimum amount of information is retained from each transaction, if only just to keep the size of the blockchain itself in control, but additionally to preserve the spirit of privacy in the system.

I noticed in the multibit wallet client there is a possibility to make a comment to a transaction. Logical. So that the recipient can put the payment into context. And so the sender can also keep records of transactions.
My question on this is whether these comments, meta data, whatever.. are also saved in the blockchain, and thus perfectly searchable and transparent!

ie. I buy a kg of smoke with bitcoin. If I write what my payment is for in the comments, am I totally defeating the idea of the privacy. Or will that comment only be tied to the wallet and not to me, I guess my privacy is still maintained as long as I do not write my name and address and such in to the comments for the transaction.

Can anyone elaborate on how this works in the Bitcoin system?

many thanks in advance for the enlightenment.

Alex