Well, yes and no.  The 3-day passes will probably sell out super-fast, within a few minutes to a few hours.  If you go to the ACL Fest website, you can sign up to get an email notification when they go on sale. This is a little hit-or-miss, too.  I've received the email up to a day late. 

The single day passes will probably be available for a few days.  Of course, these will be more expensive individually than the 3-day pass.  The good news is, even if you miss out on all of them, you can always get tickets from people on Craigslist or selling them through the ACL blog.  You do need to be careful if you do that, since these will be printpasses which means you could easily get ripped off.  However, the final bit of good news is that if you plan on coming to the Fesitival, there will always be people (sometimes scalpers, but mostly not) selling tickets outside the gates.  I've never known anyone who wanted to go who wasn't able to get a ticket somehow, and I've been going to ACL Fest for years.  There are always people who buy tickets who aren't able to go at the last minute.  Let me know if you can't get tickets and I'll be happy to keep my eyes and ears open for you.