But that is the point: there is no single "wallet address" - I use a different address for (almost) every "receive" transaction - the address used for sending could be *any* of those.
I dont get You mate.

Do You have several different addresses in the same Bitcoin QT client

Yes.
I use different "receive" addresses for different types of transactions - when money is sent to my wallet it *arrives* in one of those addresses.
When you *send* coins the client chooses one of those addresses (I think based on the coins that have been sent to it), but you cannot as the user "pre-define" which address it will use to send a given transaction.