I of course don't know if the website is showing any indication of a refresh, but it's entirely possible they'll update the CPUs with the Haswell refresh like they have with the laptops. That could be next week, or the fall, but of course may not happen at all.
If I was forced to make a bet, I think there will be a refresh, primarily because reports are that Broadwell just keeps slipping, and the desktop chips used in the iMac are now looking more like spring 2015... that would make a really long span from the last update in fall 2013.
However, it's unlikely that we're talking anything more than a 100MHz bump and maybe some changes on configuration pricing, again, very similar to the MBPs. Less likely is they update GPU options and add TB2, but it is a possibility of course.
It's probably not worth waiting for if you want a new computer now... and it will only put you in a tougher spot if you wait a few months and nothing is updated and then you're stuck between purchasing or waiting until Broadwell...
