Einstein initially added the Cosmological Constant because (if I get this right) it seemed to him that the universe should be static. I agree that back then this would have been an obvious assumption. I'm curious now, before Hubble, where there any opinions/debates about whether the universe would be expanding or contracting?
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It's a very good question but the answer is that Newton's Universe actually doesn't have to expand or contract so no cosmological constant is needed. Well, it's a bit more subtle.
The right non-relativistic gravitational equation where one should add the vacuum energy density is the Poisson equation $$ \nabla^2 \phi_g = 4\pi G \rho + \Lambda_{{\rm Newton}}. $$ I added a Newtonian cosmological constant term. For a Newtonian cosmology with a uniform mass distribution at the cosmological scale (e.g. above hundreds of megaparsecs), you actually have to add this term (with a negative sign), to neutralize the mass density at the very long distance scale. If you omit this term, the $\phi_g$ potential has to have a Laplacian with a constant term, so $\phi_g$ itself will have to contain something like $\vec x^2$ which will inevitably be minimized at some point of the Universe - $\vec x =0$ in my conventions.
Amusingly enough, one may describe Newton's gravitational forces without any $\phi_g$, by manifestly summing the forces from other point masses in the Universe. It's an equivalent approach to calculate the acceleration but it allows us to avoid the problem with the preferred point in the Universe. I may just claim that the forces acting on the Earth that are caused by very distant objects cancel. This is equivalent to saying that the Earth is the $\vec x = 0$ point - and one may say the same thing for any other object in the Universe (a method to regulate the infrared divergences from the forces caused by very distant objects).
Needless to say, the assumption that we choose a "uniform cutoff" around every probe in the Universe is totally equivalent to adding the neutralizing Newtonian cosmological constant above. Also, you won't be able to invent any non-equivalent yet consistent Newtonian cosmologies with a uniform Universe but a nonzero cosmological constant. That's really because the Newtonian spacetime is flat and the cosmological constant is the curvature of the empty spacetime - which vanishes in Newton's theory by definition.
Luboš Motl
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This is really a good question and indeed questions were raised at that time about how Newton's universe could give rise to a static universe if gravity is always attractive. Newton argued, if the universe were infinite in extent and matters were distributed more or less uniformly throughout this infinite universe then collapse could be avoided since there would be no center for the universe to collapse. However, this argument is not flawless since one can show that this kind of universe will be highly unstable. A slight perturbation can break the balance and inevitably lead to collapse of the universe. Surprisingly no body argued at that time that the universe could be expanding (or contracting). Instead some people really tried to modify Newton's law of gravity so that gravity may be repulsive at large distances. So you see, a kind of repulsive gravity was indeed proposed much earlier than GR.
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Newton used this as a proof that the universe was infinite.
If all the matter in the universe is attracting every other bit of matter then the universe should collapse into a single point. He reasoned that the only way around this was if the universe was infinite, then every bit of matter would have an equal force in every direction - from the attraction of every other object around it.
Martin Beckett
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Newton also assumed that the speed of gravity is infinite. This led him to further support his idea of an infinite universe. If he had somehow known that the "c" is finite, then he would have eventually reasoned that the universe can not be static.