I am no expert, but I have been looking around the web for just this kind of thing.
I think the easiest and cheapest way is with ElGato's Eye TV Hybrid. However, this will send the signal through your computer, but the EyeTV software is supposed to be pretty good. There is a lot of discussion about this device on Macrumors, simply search for ElGato Hybrid in the forums search. ElGato actually have several solutions for TV through a computer. They have a good reputation among Mac followers.
If you did not want to go through your computer then Gefen have some solutions, however these are expensive. You will have to spend at least $500. In fact I am not sure if the boxes you would need are still available. You need to do a search.
The problem with 30"ACDs is that they only have a dual link DVI connection ( I think this is correct), so, if you want to connect your Sky+ box directly you need another box to change this connection to whatever your Sky+ box has. In some cases you also need an analogue to digital converter also. (not sure if this will be the case with Sky). If you google you will find many forums discussing exactly your question, i.e. attaching various TV boxes, games consols etc. directly to an ACD.
You may come accross some issues involving reolution, HD comaptability
h.264, 1080i etc. I tend to get a bit lost when people go an about this stuff. But, again, search just to make sure you can do, and see, exactly what you intend.
If you do not already have the 30" ACD you may want to look at the Dell 30" LCD, as this has more inputs than the Apple offerings, so you can connect directly with no additional hardware. The Dells have quite good feedback.
I've never seen a solution for the Mac or the PC that allows you to connect an RGB Scart source to the computer. They only ever offer S-Video (for our American cousins) and RF (which will be very poor in comparison to RGB Scart). So basically you can't connect SKY to your Mac without sacrificing the image quality.
There might be an (expensive) box you can buy from someplace (perhaps keane electronics) which will convert the RGB SCART signal to DVI so that you can at least connect SKY directly to the ACD and perhaps switch between it and your Mac without ever using the Mac as a PVR (which you'd not want with SKY+ anyway). But this option is likely to be at least a couple of hundred quid.