When I am holding my electric guitar
when it is plugged into my guitar amp, and I attempt
to touch my computer (to start recording, for
example) I get an intense static shock.
Jack,
There may be no solution. It could simply be your electrifying personality. But, barring that....
A static shock is less indicative of a poor electrical grounding scheme than if you had, as others have described, a constant noticable current when you touched the Macbook.
As you strum your guitar, you and your rig are building up a static charge which likely has nowhere to go because your amps input scheme doesn't tie your instrument cable shield to earth ground. Even if it did, you and the strings may not be connected to it, so the amps grounding may have nothing to do with it. You may notice it more when you're plugged into the amp because that's when you're "strumming".
Strumming your guitar and then touching the Mac is analogous to shuffling your stocking feet on the carpet and touching a metal doorknob.
This is less hazardous to you than it is to your computer and its data. But there is a solution. Check these out:
Something for the table:
http://www.apogeekits.com/anti-static_mat.htm
Something for the floor:
http://www.uline.com/BrowseListing1755.asp?pricode=wf394
You can get a mat for the floor that you simply step on before touching the Mac, or you can put a mat on the table and touch it before you touch the Mac. Setting your Mac right on it wouldn't hurt either. The mats provide a resistive path to ground that slowly and safely discharges the static charge. So, no more shock.
Regardless of what you do, be sure you have a good ground available and that everything grounds to the same place. Hopefully that's 2 copper rods driven into the ground somewhere outside your studio.
Many products, probably your Mac too, do not have a grounded case. This isn't a bad thing. But, like the doorknob, they can still have enough of their own ground potential to discharge a static charge.
Good luck Sparky.