Notch only mentioned traveling to a city where Valve is also located. Nobody ever mentioned them having contact, it's just rumours.
And besides, Minecraft's is a Java game. I can assure you that Valve does not and will not have Java programmers working for them.
The only viable possibility is Valve investing in Mojang, but there's no reason to think such a small team of developers needs more than €10 million to finish Minecraft.