How to attach knock off marble top to tulip base?  

Wood, maybe.
If you find a Knoll base, it'll have a central threaded stud that expects to screw into a threaded hole in the center of the top. No gluing opportunities there unless you cut the stud off and grind everything smooth. And even then, it's a pretty small-diameter mounting surface with not a lot of surface area for a secure bond.
If you find a Burke base, it'll have a plate with four holes around its edge; it expects to be attached to a laminate top (Burke didn't do marble, as far as I know) via screws through those holes. I THINK the plate is flat, so maybe you could glue directly to it... But if it were me, I'd probably fit a round or square wooden plate to the base, then glue that wooden piece to the marble top. The wood would have more surface area than the base's mounting surface, so the adhesive bond would be more secure, plus the wood+marble top would be removable if you ever wanted to move it to another base or pack it for shipping.

I agree --
In my experience, marble table tops are never permanently affixed, they just sit upon the base. But, most table bases are more substantial than a tulip base.
Seems the thing to do would be to attach a metal plate (slightly smaller than your marble top) to the base, to balance the heavy marble.
How you're going to do this, I can't say, without seeing the base you choose. (Offhand, I think some welding might be involved. Even then, I foresee problems in making the metal plate flush with the top of the tulip base...)

To clarify...
I wasn't suggesting that the wood+marble top should just rest on the base; I meant that the wooden plate could be drilled for the Knoll base's central stud or for the Burke base's four screws, so the top could be securely affixed via those fasteners.

awesome suggestions.
Thank you both for the brilliant suggestion. I think having the wood plate inbetween the marble and base is the way to go. Now, the only trick is finiding a burke/saarinen or other knock off base. suggestions? I'm been trolling ebay and craigslist, but haven't had much success in finding just the base. Thanks again!

Finding a base
It's rare to see a base without a top on Ebay, but complete tables with beat-up laminate tops are fairly common and might be relatively inexpensive.

PC-7 Epoxy paste-
I should think this would do the trick,without drilling,mounting special plates,etc.

Tulipman
I don't think there's enough flat surface area on either the Knoll or the Burke base to provide a secure adhesive joint directly between it and the marble top.
The intermediate wooden layer solves that problem by providing a large flat surface on top for glue, and an easily drilled surface on the bottom for either the Knoll stud or Burke screws.

There's a marble-topped Saarinen on Ebay now--
How do you suppose the top's attached? Or does it just balance there? Were the tulip bases modified for the marble tops?
The base shown (see link below) seems to flare out more than usual-- just what you'd need for a marble top.
http://cgi.ebay.com/NR-SAARINEN-BASE-MARBLE-TOP-TABLE-MODERN-EAMES-ERA_W...

That one...
That one looks like a Burke base that's been married to a marble top not originally intended for it. All Burke bases have that wide flare on top, just as all Knoll side-table bases have the same narrow flare whether they're attached to stone tops or wood/laminate tops.
Hard to tell how (or whether) the top and base are attached; it's unfortunate that the photos show every angle except the one from below that would make it clear.

And if you like that one at $200...
... then you should LOVE this one at $250. Still not Knoll, of course, but brand-new and with a significantly nicer-looking marble top.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180407925168

Knoll Table Base
Did you ever find your Knoll table base? I have two! If you're interested let me know!

wmrosman, I know this is dated but, do you still have the bases?