I used my IPad2 AT&T sim and got the invalid sim message.
Is it possible that you got that message because an iPad micro-SIM won't work in an iPhone and vice-versa? Regardless of whether it's locked or not, they're programmed differently.
I used to be able to pop my iPhone 4 micro-SIM into my iPad 1 running iOS 3, change a setting, and use my phone's data plan and surf the web on 3G. With iOS 4, Apple disabled that. I'm not sure what message pops up when you pop it in, but I know it won't work.