No you can't with the native iOS calendar. If you try something like Good for Enterprise that does support showing availability, but is obviously an add-on with costs both in terms of license/support and actually deploying it/maintaining it.
Shame that Apple still hasn't deployed some basic functionality in the iOS to rival the blackberry support of Exchange calendars. It would certainly speed up the corporates to swap from blackberry to iPhone/iPad.
Check out Scheduler (http://itunes.apple.com/us/app/scheduler-by-superconnect/id546857260?ls=1&mt=8), it allows you to find common open meeting times and schedule directly through your Exchange account on iOS.
The lack of free/busy calendar support for Exchange is truely astonishing. We are currently undertaking a feasibility study to consider using iOS devices in place of Blackberrys. This feature is fundamental to the business user. How can Apple expect to make inroads into the corporate space if they miss something as obvious as this?