Nokia is said to be getting Instagram as an exclusive app on June 26. Windows Phone has lacked many popular apps from other platforms, this has been seen as a problem for Windows Phone adoption and one that Nokia has decided to address. If this rumor proves true, it would appear that Nokia's work has paid off.
Exclusivity seems just slightly unlikely as Instagram would probably not want to cut themselves off from a greater market possibility. However, it could end up being an exclusive for a certain period of time. Either that or Instagram doesn't see the Windows Phone market as being that huge. After all, Nokia is one of the major Windows Phone players, so they would still pick up a large portion of Windows Phone users.
We'll have to wait until an official announcement or June 26 to see if this rumor proves to be true. Until then, take it with a grain of salt.