I have wondered for some time about Informatica’s intentions in the MDM space. There had been some market rumours about them possibly buying Siperian, but it seems as if their not so secret meetings with Siperian were actually to form a partnership, which was just announced.

This is an eminently sensible partnership in my view. In most MDM projects there is going to be much gather of data from multiple places, and so MDM vendors typically need to rely on integration, or at least data movement technologies that enterprises have already deployed. Assuming that this relationship will be exclusive (unclear from the press release), Siperian has locked in a relationship with the last remaining independent ETL and integration player of note (Ab Initio is also still out there, but is so secretive that it is part vendor, part cult). Informatica gains a foothold into the fast growing and important MDM market, so it is a win-win as far as I can see.
Perhaps one day this relationship will lead to something more binding, but for now these two vendors appear to be dating rather than tying the knot.