Proposing collaboration to an invited speaker? I am organising a conference. This will take place in 6 months time. One of the invited speakers works on a topic that is highly related to my research interests. I would like to propose a specific collaboration to this professor, but I feel that the fact that I invited him to the conference (I am paying for his accommodation, expenses ...) may induce some pressure to accept my invitation to collaborate.

I wonder whether I should just wait after the conference to tell him about this project or if it is OK to tell him now? The reason I want to tell him now is that I would not like to wait for another 6 months to start this project.
I don't see any ethical dilemma here unless you threaten to retract his invitation. Don't even hint at it. You liked his work enough to invite him to speak, you should feel free to invite him to collaborate. If he declines, thank him for considering it and tell him you're looking forward to his talk this summer. If he shows interest in collaborating, keep the email conversation going, set up a phone call, etc, etc, and see where it goes. You might be able to use his visit to get some real work done (write a paper, write some code, write a grant, etc) if you're already agreed to collaborate. Don't waste your time between now and then.