According to Barack Obama website, 27.5 delegates to go.
The following post on the blog at BarackObama.com, Obama now needs just 27.5 delegates to make it official. This may be a conservative estimate given that the AP has already declared him the nominee. I am of the opinion that the race will be over tonight. Keep it here for the latest information that we can find.
Here is the link to the blog post: http://my.barackobama.com/page/community/post/samgrahamfelsen/gGByq8