As announced on Google+ earlier this morning by the Android Developers page, app devs can now distribute their [free] wares to users in Iran. Paid apps and all apps with in-app billing will have to wait, possibly indefinitely. The complex and restrictive embargoes the US has placed on doing business in the country, particularly when it comes to accepting Iranian currency or working with Iranian financial institutions, probably are something of a hurdle in that regard.
It also doesn't help that the government of Iran has notoriously tense relations with Google. As of 5 months ago, Iran's leadership once again blacked out a number of Google properties as part of the country's increasingly oppressive internet restrictions. This blackout appears to still be in effect. Many Iranians have continued to work around these restrictions illegally, though, and Iran's government has historically had difficulty staying true to its word on the whole "Google issue."
Regardless, free apps can now be distributed in Iran to those people who are able to get to them. And in a country where the average person's internet is forcibly restricted to a speed of 128kbps, increased access to lightweight mobile apps is probably more than welcome.