We have optional Battle.net conversion right now, but that will be mandatory at some point in the not-too-distant future.Many people have done the conversion already, and many speculated and inferred from past Blizzard comments that the conversion would eventually become mandatory. But now it looks like it's going to be, and that date is in the "not-too-distant future."
The conversion process is relatively straight forward, and can be started by visiting your account management page. There have been times when Battle.net enabled accounts were not able to get into the game due to authentication issues, but those seem to be diminishing.
We'll be sure to keep you updated on any additional announcements concerning the upcoming mandatory conversion