It's not possible. Doing a bit of searching I ran into this blog comment that had some text from a Google Support Email reply
“The underlying issue is that the system is not taking 12am-12am as valid operating hours” and “After speaking with our technical team members, your hours may or may not be listed as 12am to 12am after the next index push. There is the option not to display hours, which may be better suited for a 24 hours business, such as yourself.”
The blog comment was made on 1/6/2012 so this is a timely answer (hopefully they fix it).
A Possible Solution is to add to the
Additional Details section with
Open 24 Hours:
Just for kicks I'll throw in the link I found that through since it lists bugs known in the Places and Places Dashboard.