AT&T has started to distribute the iPhone 3G to businesses. If your company has already a business contract you can directly order your iPhone 3G on the AT&T website.
If you plan to actually use the access corporate email, company intranet sites, or other business apps you need the Enterprise Data Plan for iPhone that costs $45/month. If you just check private mail then you only need the normal Data plan for $30/month.
If the iPhone 3G is ready to be a full blown business phone is highly dependent on how you want to use it. If you are a hardcore Blackberry addict and type emails blind under conference tables forget the touchscreen iPhone.
More details on the AT&T Wireless Business Center site.