So, if I read the news correctly, I can either start receiving my 3 GH/s share from the data centre operations right away (as soon as it's officially operational), or to wait for the Neptune to get developed and have it hosted at the aforementioned data centre? Correct?
Sound like a good option to me. I have a rig hosted with them now, and it's been operational 24/7 since it went on-line. So I am happy with their handling of the hosting. When I bought Neptune, it was at the limit of my power and, not least, cooling abilities at home, so I was actually asking KnC if they would provide hosting for Neptunes. It seems they listened.
Currently, I don't see any downside...
I think you are reading it wrong.
They are building the data centre shortly with their 28nm products. That data centre will be raise the difficulty dramatically and reduce your current income (and the future income of the neptune).
Once it gets to July 1st, the first day they will be 'late' THEN you get access to your 3TH of hosted mining, BUT at that point it will probably be making 0.01 BTC per day and any chance of an ROI will be gone.