I have an iPad 2 and I need a device to test my ios app on. I was wondering if their are any consequences to registering a device to be a developer device. For example, can I still use it as a regular device when I am not developing on it? Will I be able to use it regularly if I put iOS 7 on it?
Still works fine. This will allow you to install you own Apps on the device for testing. Otherwise the only way's to get apps on you iOS device is Jailbreak or App Store.
Registering an iPad to be a developer device just means that you can install your own compiled apps (which were built specifically for your device(s)), and run them. You can run "normal" apps, acquired through the iTunes store, or any apps which were compiled for your iPad.
WRT iOS7, be sure to read Apple's pre-release disclaimers. (There's a section "Read Me Before Downloading" on the developer site). Warnings about putting your device "in an unusable state" are there for a reason. Unless you are writing apps that make specific use of new iOS features, you can still test new apps in iOS6. And, the simulator is also a useful tool.