Can you use a blackberry 9900 without a data plan?

Yes!

What you'll have to do is buy the blackberry for the full price, no contracts. Then, depending where you live - you can buy a SIM card from the phone company of your choice (If you live in Canada, only Rogers offers SIM cards). When you buy it, they'll help you to set it up on Pay As You Go, or you can put it on your own plan - but unfortunately the Blackberry E-mail service wont work - but if you're looking to go on a plan, check in for the rates on Data - that way you can use the net on the phone.

Right now I have a Blackberry Pearl 8100 and it's set up on PAYG - it's great. The only problem is that Data on PAYG is expensive if you're using facebook or MSN.

Hope this helps!