This has been well-addressed by the answerers before me, but just to chime in -- there are a variety of analogs one could make for the Poincare conjecture for number fields.  For one, there are several equivalent statements about the Poincare conjecture for 3-manifolds which are *not* equivalent when transferred over by analogy to the number field case.  As a first easy example, while 3-manfiolds enjoy a clean Poincare duality, number fields have extra 2-torsion.  In particular, one frequently has $H^1(\mathcal{O}_K,\mathbf{G}_m)$ trivial with $H^2(\mathcal{O}_K,\mathbf{G}_m)$ non-trivial (example: any real quadratic number field with trivial class group).  The equivalences (or lack thereof) between being an integral homology 3-sphere, a rational homology 3-sphere, and a homotopy 3-sphere are not the same in the two "categories."  So depending on how you phrase your analogous Poincare conjecture, you may get different answers.  The cleanest form (found in Niranjan Ramachandran's "A Note on Arithmetic Topology", which deals exclusively with this question) is that there are exactly ten rational homology 3-spheres which are homotopy 3-spheres, namely the 9 quadratic imaginary number fields of class number one and $\mathbb{Q}$ itself.  (Or really, $\mathbb{Z}$ itself), and even more homotopy 3-spheres.

A second frequently under-emphasized point to make is that no one *really* knows what the right category for this analogy is on the number theory side.  As mentioned above, if you take your category to be Specs of rings of integers in a number field, you don't get the Poincare conjecture.  On the other, if you take the point of view of Artin-Verdier theory (or alternatively, Arakelov theory), where you include in your spaces some information about the behavior of the infinite primes (from the point of view of number theory, defining Spec(Z) as the set of prime ideals ignores the obviously important primes at infinity), then you get a different cohomology theory.  With these new cohomology groups in place, some things look a little bit cleaner.  Again, see Ramachandran.