if i'm on the iphone enterprise developer program for companies with >500 employees, do i still need to publish my app on the app store for clients to install the app?
If you do not intend to publish apps to the App Store, then don't. The iPhone Enterprise Developer Program is designed not to allow you to publish apps to the App Store. For that you need to also enroll in the standard program.
From Apple Developer's FAQ for the enterprise program:
If your company is enrolled in the Enterprise Program but would also like to distribute your application(s) to customers via the App Store, you will need to enroll in the iPhone Developer Program as an Individual or as a Company.
think i found the answer here http://stackoverflow.com/questions/1509513/does-the-iphone-ipod-touch-support-bulk-enterprise-app-deployment-or-would-all
The iOS Developer Enterprise Program is now open to all companies - see http://developer.apple.com/programs/ios/enterprise/ - they removed the 500+ employee limit. This will definitely help companies that want to roll out internal "in-house" apps.