It's a common dictum, derived from over-reading Hume, that 'is' does not imply 'ought'(INO). But it is also a common dictum, derived from Kant (I make no judgment here about whether it is Kant over-read), that 'ought' implies 'can'(OC). It's notable that these two dicta are inconsistent with each other:
(1) If X ought to do Y, X can do Y. [OC]
(2) If it is false that X can do Y, it is false that X ought to do Y. [(1), Contraposition]
(3) If it is false that X can do Y, X is lacking in the ability to do Y.
(4) If it is false that X ought to do Y, it is true that X ought to do something other than try to do Y [as if Y ought to be done].
(5) If X is lacking in the ability to do Y, X ought to do something other than try to do Y [as if Y ought to be done]. [(2), (3), (4)]
But (5) tells us that an 'is' implies an 'ought', which violates INO. But (3) is clearly a tautology, and (4) is clearly true for anything to which 'ought' can apply at all, assuming that you ought not to try to do things that you ought not to do; which is arguably a tautology itself.
This is a general issue. The reason someone like Hume can accept something like INO is that he has a very restricted and minimal account of modality (i.e., necessity, possibility, etc.). Anyone with a more robust account of modality than Hume has to allow for the possibility that they can, in fact, get an 'ought' from an 'is' that has these more robust modalities. At least it would have to be proven on a case-by-case basis. An 'ought' is just a strong modal operator. It can indeed be difficult to get from a situation that has no well-defined modal operator, or a distinct and isolated null modal operator (as 'true' sometimes is), to one that has a strong modal operator. But it is not generally a problem to get from one well-defined modal operator to another; it just depends on the modal operators in question and how they are related.
Incidentally, this kind of modal situation is not uncommon. Kant has a more robust account of modality than Hume, but almost all of Kant's more skeptical results follow simply from the fact that for Kant all modalities are in one way or another epistemic, having to do entirely with how we know things. If you have a less restrictive account of necessity and possibility than Kant does, you can no longer assume that Kant's conclusions follow from his principles -- it would have to be examined on a case-by-case basis.
[ADDED LATER: A phrase accidentally was dropped out that made (4) read oddly, and this mistake was propagated through the rest of the argument. I have fixed it.]
