Why is my filtered index being ignored?
Since SQL Server can skip NULL rows to start the range scan, the cost of either index is identical, so this is basically a coin toss for the optimizer. Look at the plans in SQL Sentry Plan Explorer* by default and when you hint the index (click to enlarge):




Since it's a toss-up, I don't know what benefit you'd get out of forcing SQL Server to choose one of two equally valid options.
* Disclaimer: I work for SQL Sentry.