Can a AIS lens be adapted to a Nikon digital or not?

There's no need to 'adapt' it to digital. But you are asking a fairly broad question without giving much information. Check your manual to see which ones should be used and which ones shouldn't for your particular camera and or lens.

If it's a D200, you can use the AI-S lens without modification, but you have to program some data into the camera.

If it's a D70 or D80, you can use the AI-S lens in manual mode.

I don't think you can use it on a D40 at all. I think the D40 and D40x require AF-S lenses.

It's generally the pre-AI lenses that need modification. Here's a place that does it:
http://www.aiconversions.com/

You can "use" AI or AIS lenses on any modern-day Nikon DSLR. This includes the D40 and the D40x. All Nikon cameras will mount the lenses w/o damage.

The following cameras will require operation in fully manual mode:
D40, D40x, D50, D70, D70s, D80, D1X*, D1H*

The following cameras will work in both manual and aperture-preferred modes (ie the camera will meter on the lens after you input the focal length and max. aperture via a menu option [on D200 it's called non-CPU lens]):
D200, D300, D2H, D2Hs, D2X, D2Xs, D3

* I'm not sure whether these ones will meter as well.

The D40 acts like a D70/80 in regards to AI(s) lenses. The D40 is the only Nikon digital that can also take Non-ai lenses, due to the design of the AI sense pin. The D40, D70, D80, and (I think) D100 will take manual lenses, but there will be no metering, unlike the D1/2/3 (including the D1x and D1h, Avi ) and the D200/300.

AND....to add more relevant information to this thread, you can have a CPU chip added to manual focus lenses in order to allow metering with the DSLR bodies that don't already allow this (e.g., sub-D200 bodies). But good luck these days because the folks that offer this service are getting few and far between.
http://www.naturfotograf.com/lens_CPUconversion.html

~Ted

Thanks dirb9! Now I know.
Metering can be done via histogram on the lower-end bodies. Or by using the old sunny-16 rule of thumb.

Thanks to everyone here. I'm currently a Canon guy but I've been looking on this side for better ergos and opportunity. I came from Nikon and still own several ai and ais lenses and would consider the investment if it seems worthy. Particularly if I could use them without a $200 dollar adaptor. I'm afraid if I go look at a body I'll come home with a lighter wallet.

Thanks Again Folks!