You may want to browse Alteras PCNs http://www.altera.com/literature/lit-pcn.jsp#pdn. I know, that some ACEX1K devices have been already discontinued in the past. If you are supplied with these parts by a qualified distributor, you can expect to be informed before a last buy period expires.
The fact, that ACEX1 isn't supported by Quartus II V9.1 may indicate a limited lifetime of remaining ACEX types. If they are still needed in quantities, Altera will probably continue production. In the past, the closure of antiquated chip fabs has sometimes forced a products end of life, of course, this may happen again.
In my opinion, still using ACEX series in active products today is first of all a huge waste of money, of course you have to find your own tradefoff with redesign costs.