Hi chromodoris,
I lived less than a mile from a small windfarm for seven years, and just down the road from the UK's largest on-shore windfarm (Cefn Crois).
I gather you're really interested in the archaeological repercussions of the development rather than an environmental debate, so I'll try and stay focussed. Here's my perspective.
If it weren't for the access roads, cabling and sub-stations, windfarms would have very little negative effect on their immediate environment. Unfortunately access roads in particular cause a lot of problems. They're essential to get the turbines built, and later on so that turbines can be maintained (although it's the building that makes the roads so darned large).
The larger windfarms require very solid access roads, and given half a chance the developer will make these double width to speed up construction. Apart from the risk that road material will be quarried locally, destroying undiscovered archaeology in the process, the main problem with the access roads is their effect on the hydrology of the surrounding landscape. Large areas can dry up or become water-logged if enough care isn't taken.
It's unlikely that direct damage will be caused to a known prehistoric site by windfarm developers. Damage to a site caused by a change in the hydrology seems possible, but I don't know if this has ever happened. In general the developers have enough of a fight on their hands arguing with locals about the aesthetics, so they don't try and make waves unnecessarily.
As for (as yet) undiscovered archaeology goes, I wouldn't expect anything that poses a threat to the development to survive. It's tricky to spot a cairn when you're digging with a JCB. Even if they did spot a site it's likely to get brushed under the carpet to save some money (developers are not generally in it for ethics alone). This aspect of windfarm development is no different to any other type of construction. Don't trust them. They will lie to you!
Believe it or not, I'm pro-windfarms.
:-)#
K x