Question: If I am messing around with XCode and trying to learn Objective-C, does it pose any threat to my system if I routinely forget to release objects? What is the real consequence of not releasing objects? If I restart my computer will these memory leaks persist?

I am mostly messing with NSStrings and NSTimers and things like that, nothing serious. I'm just concerned about it messing up my computer.

Is there any way to check if there are unreleased objects? What is one to do if they want to check a bunch of code to see if there are any of these memory leaks?

Answer: 
The only "threat" is that your program will eat up a large chunk of your available memory, which will cause your computer to go slow and your hard drive to thrash until the system reclaims the memory when the program ends. It makes things pretty unpleasant when a program uses more memory than it has to. Not dangerous — just really annoying.
No, you can't hurt your machine by forgetting to release objects. There's very little that you can do to hurt your machine in software short of deleting system files.

As others have pointed out, all memory used by your program will be reclaimed when the program terminates. If you want to check your program for leaks while it's running, you can monitor it with the Instruments tool that's included with Xcode. I won't try to explain how to use it here -- read the docs and ask if you have trouble.

You can also get some clues that an app is leaking memory by monitoring its memory usage with something like the command line `top` program. If an app's memory usage is constantly increasing as you use it, that's an indication that there may be a leak.