I went to update Thing 2’s (that’s how I refer to my male child online – his female twin is Thing 1, and the youngest is The Kidlet) Nokia Lumia 900 to Windows Phone 7.8 today, and the Zune Software told me there were no updates available. After a quick search online, I found a few articles indicating that Microsoft pulled the update sometime within the last day or two, after being available for just under a month.
I haven’t seen any official word from Microsoft, yet, explaining why the update was pulled.
Unfortunately, if you haven’t already updated your Windows Phone 7 device to the new version, you seem to have missed the boat (temporarily, at least). Hopefully, Microsoft will get this straightened out quickly, and the carriers won’t hold things up too much when MS gives it the go-ahead again.