5530 card is unlocked on US supplied mini9's. I origonally submitted unlock code request to AT&T per older posts, and they sent back reply card is not locked. I use T-Mobile prepaid sim in Texas, and it works fine once you configure connection correctly. The default connections in the Dell Wireless Manager program are set for datacard accounts, so if you wish to use phone type sim account with data, you have to enter correct APN (i.e. wap.voicestream.com for T-Mobile). I've also successfully set up AT&T prepaid GoPhone type account on the sim that came with the Mini when purchased from Dell. If you need info on how to do this, PM me. AT&T prepaid however is expensive at 0.01 Kb.
For those who wish to try test w/o getting prepaid contract type account, the least expensive way is to buy a sim card from T-Mobile web site for $6.99. Then go on ebay and purchase 1 month unlimited T-Mobile activation for apporx. $5. The card will then give you unlimited data, SMS, email access via T-Mobile for approx. 30 days before you have to either sign up for account, or trash card and buy another.
Hope this helps anyone like me who just wanted to test the data card in my Mini w/o having to sign up for contract service.