Yes Protoshares can still be mined after the initial Bitshares chain is released. Bitshares on February 28th is only the first chain, more chains and DACs will be released after that for a long time.
Protoshares will have 2 million total and only 1.5 million have been mined. I also believe after the 2 millionth Protoshare that is mined there will be 1% per year additional that can be mined (I believe this is true, but not 100% sure).