If this holds out to be true and not just a trial balloon you can expect the shit to hit the fan fairly soon, unless, that is Russia really is full of bluster:
Ukraine has agreed to take part in a missile defence system designed by the United States to protect Western countries. The government in Kiev defended its decision for military co-operation with the West, saying Russia cancelled a bilateral treaty with Ukraine earlier this year.
A few days ago, Poland and the United States reached agreement on the siting of missiles on Polish territory. These, together with radar installations in the Czech republic, make up the missile shield. Russia is fiercely opposed to the defence system and has threatened retaliatory measures.
The Ukrainian offer to co-operate with the US on the shield comes as the situation surrounding Russia’s military operations in Georgia is increasingly tense. Ukraine’s pro-Western President Viktor Yushchenko has strongly criticised Russia and is threatening to impose restrictions on Russian navy vessels’ use of the port of Sebastopol in Ukraine.
I just can’t see any situation where Russia would let this stand.