Windows Phone 7: Professional Tips - Double tap
When you want to make sure something is dead (finished), you shoot it twice—also known as a double tap. Windows Phone 7 supports this too… for sentences.
When typing, double tap the Space key, and it will insert a full stop and a space!
Considering the size of the Space key and how easy it is to double-tap it, rather than find the smaller dot key and move away from it, it can save you a little time.
This is just one of the many awesome UX features in the phone!