Thanks for the replies!
Jim - Thanks! Unfortunately, I don't think this will make a difference, since it won't work even with the microphone built into my iMac (which I have tried).
Chesterr - It's 5.5.0. I checked for updates, but my Mac didn't find any...okay, I just googled it, and it looks like there is a version 5.5.2. I will download that and see if that fixes anything. I have to admit, the rest of your post is a little above my know-how. I had to look up "Mac root account" to even know what it was, lol. I suppose I'll look into that more if this update doesn't work.
As far as updating the application is concerned you can update it from within the program menu.
Go to help in the menu bar then click on updates.
Adobe Application manager will automatically find the relevant updates.
Make sure once the updates are downloading you have to quit Premiere Pro.
BTW here are the instructions on how to enable a root account.
So, the update didn't change anything. To be a bit more clear about what's happening, here is a video of basically exactly what I am doing: http://www.youtube.com/watch?v=jaeIcvHjaug
It looks like he's using a different version of Premiere Pro, but essentially, that's what I'm doing in a nutshell, and it worked exactly like that for an hour. Now all of the sudden, when I click the little red record button, instead of holding a nice steady red, it blinks on and off, my track records for maybe 6 seconds, and then it stops recording. Premiere recognizes my microphone without any issues. But I can't get it to record for any length of time now, despite the fact that it already recorded an hour's worth of commentary for me just fine, and I haven't changed anything.
Thanks everyone for all of your suggestions. Unfortunately, the issue is still unresolved, but we went ahead and upgraded to CS5.5 Master Collection, moved the project to Audtion, and were able to finish the commentary track there. I'll have to see if the original problem happens again with the next project in Premiere Pro, but for now, we found a workaround.
I have searched everywhere and tried everything I can think of. Restarted Premiere. Uninstalled/reinstalled premiere. Shut down/unplugged/replugged in C-1 microphone. Updated software. Restarted iMac. Nothing works. I am out of things to try, and hoping someone here might have a suggestion.
You're not done troubleshooting until you repair permissions, rebuild your directories with Disk Warrior, zap the PRAM, and run utilities on your hardware and RAM (TechTool, memtest/HighLoad, etc.
"A serious error occurred" suggests to me a harware or OS issue, esp. since un- and re-installing the app didn't fix it. Did you delete the Pr prefs when you uninstalled?
Audition may have worked because it's 32 bit, and Pr is 64. Mull that over a bit, and maybe you'll have a revelation.
Good luck and glad you found a workaround.
What OS are you running, BTW?
Now Adobe Premier Pro is just crashing repeatedly. I don't do anything at all. Open Premier and it crashes. Restart the iMac and it crashes. Reopen and it crashes. It's like a terrible loop and I can't do anything. Every time I open it now it crashes.
Process: Adobe Premiere Pro CS5  Path: /Applications/Adobe Premiere Pro CS5/Adobe Premiere Pro CS5.app/Contents/MacOS/Adobe Premiere Pro CS5 Identifier: com.adobe.AdobePremierePro Version: 5.0.4 (5.0.4) Code Type: X86-64 (Native)
Parent Process: launchd 
Date/Time: 2013-08-29 11:44:31.601 -0700 OS Version: Mac OS X 10.6.8 (10K549)
Report Version: 6
Interval Since Last Report: -2507017 sec
Per-App Interval Since Last Report: 97 sec
Per-App Crashes Since Last Report: 5
Anonymous UUID: 6C9EEA26-1796-4F96-B35B-6E6E790853E0
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_PROTECTION_FAILURE at 0x00007fff5f3fffc8
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
So, I came here looking for answers to this problem, as is Adobe's recommended way to get support. Microphones don't seem to be working within Adobe Premier, which is more than a minor inconvience because it makes it impossible to do voice overs.
Again, aside from scolding me for posting my crash log, which I thought might be helpful, do you have any recommendations? work arounds? It seems others have this problem and it's persisted for over a year and a half. Not to sound whinny, but it seems like a fair question to me. I'd love to hear any ideas and suggestions that might resolve this.
Your very first post suggests that we use a mic that connects via the Mic port, not USB. But as others have pointed out, it doesn't work. None of the Mic options work, and yet my Mic is recognized by every other application, and the OS. So with all due respect, that's not a helpful response.
How does one get Adobe's attention for something that is an obvious bug in Premier? This is a bug, and a very obvious one, and it's lasted for over a year with no help from Adobe.
as others have pointed out, it doesn't work.
No one has yet tried that suggestion, at least not in this thread. All that has been listed as being tried are USB mics and the built in mic, neither of which is my suggestion.
Connect a microphone to the 1/8" mic. input (pink) of the motherboard or internal sound card.