So, it would seem that Russia did violate this agreement but there remains to see what other countries will do. One thing to underline is that they would have to respond to a call for help from Ukraine and, with the ousting of the Ukrainian government, it is still unclear who is to make the call.
The strategy of Russia might just have been to play on this period of uncertainty to claim some of Ukraine's territory. Because the US-Russia relation is still similar to the one from during the Cold War, they might really want to avoid a direct conflict with Russia. Until the situation in Ukraine is settled, Russia can move its troops without having to face the US. Once it's settled, the US will probably not want to engage a fight and will not push them back out of Ukraine.
tl;dr: Russia might be on a conquer-and-hold over Crimea
Edit: the agreement is the Budapest Memorandum on Security Assurances [2]
[1] https://en.wikipedia.org/wiki/Nuclear_weapons_and_Ukraine
[2] https://en.wikipedia.org/wiki/Budapest_Memorandum_on_Securit...