Just in case June wasn't busy enough for mobile app developers, between Apple's WWDC and Google I/O, Microsoft has thrown its hat into the ring. It's scheduling a Windows Phone Developer Summit in San Francisco for June 20th and 21st, just a week before Google's meetup. Details are scant in the notice Engadget received, although Microsoft teases us with the prospects of learning "developer opportunities and platform capabilities in Windows Phone." Whether that means Apollo or just more about Windows Phone 7.5 Refresh (Tango) is still very much a mystery. Either way, it's likely to be good news for Metro-friendly developers crestfallen after MIX was shut down.