As I understand it, a pruned node still has the UTXO set (this is the database of unspent transactions, keyed by txid, that is necessary for knowing if tx's that come across the client are valid and should be relayed or not). Assuming Armory is written to support it (not yet, I'm sure, but it could be), you can still sweep/import a private key, and receive and spend bitcoins. The biggest difference is that you won't be able to know the history of old, already-spent transactions. Also, this might be an issue as Armory may have a harder time figuring out how many addresses to generate from your wallet, if it doesn't know that the first e.g. 150 addresses are already used and spent from.