What version of Premiere are you using?  6.0 or 6.0.1 or 6.0.2 or 6.0.3?

Also sometimes this problem occurs when you used a customized sequence in the project but this sequence/codec is no longer installed is this possibly the case? For example did you have any capture cards installed that aren't installed anymore? Did you recently re-install your OS or remove codecs or make any system changes?

Also from what I have read another quick work-around for this issue is to open the Premiere Pro Project up inside Notepad ++ and change all instances of the current preview codec from I-Frame Only MPEG to Microsoft AVI. Because some other people have said this solution works in some cases. However I'd only do this as a last resort. This solution will only work though if the problem was being casued by the I frame only mpeg preview codec option though which isn't always the case. So this might not work anyways. I'd honestly recommend just re-installing though if you've already tried to deactivate and you personally already know that you weren't using I frame only mpeg as a preview choice and you also know for a fact you haven't uninstalled any capture cards recently or any codecs. If you do uninstall though, use the adobe cleaner tool when doing so.

http://www.adobe.com/support/contact/cscleanertool.html

Glad to hear it worked, looks like this problem has been solved!

Sadly though I don't know what caused it, other than something has obviously gotten corrupted. I'd recommend uninstalling and then using then adobe cleaner utility for a perm fix.

Well yes and no, we know what the problem was now so, in that since it's solved. Sadly though like you very correctly stated it's not a perm fix at all. Which is why I recommended a clean install for the perm solution. So the problem has been solved, a solution hasn't fully been provided yet. Only a suggestion on how to get there.

You're correct the source hasn't been solved. Only the cause disovered. So I guess I should have technically said "looks like we have solved the cause of this problem". 

(Even though that is not technically fully solved because the cause of how exactly it was corrupted is still unknown)

Like I mentioned earlier you're going to need to uninstall the suite, then run the cleaner utility. Then reinstall, unless someone else knows how to fix the issue that caused the corruption in the first place. Which I have no clue how it managed to get corrupted, I just know that somehow it did. That's why that problem is solved (meaning knowing what exactly the problem was), since we know it's corrupted, sadly though I still have 0 clue why.   I'd really just recommend the uninstall option. It's by far the fastest solution IMO because you could end up spending hours/days trying to work around a issue that can be fixed by a fresh re-install (most of the time). I've fallen in this trap myself before.  

Just make sure to follow the steps listed from the link I posted in my earlier post.

If you really want  avoid a fresh install though. (Which I'd strongly recommend doing instead of doing what I'm about to say)

You can technically create a customized sequence that is using a different codec for preview, then paste your old sequence into the new sequence then delete your old sequence which was using the I-frame only mpeg for previews, then click save as  and name the project w/e you want then use the project which only contains the good sequence which is using a different/ your newly selected preview codec.

Additional info, I have tried in CS6.0.0 & .0.3, and both on my Macbook Pro & Win machine. Moreover, since uninstalling/reinstalling, I cannot access my deactivate or update buttons in any of my CS apps....

I-Frame on the Mac, but I-frame is gone on my PC. If I could just get I-Frame on the win computer, I would be gold. I have been able to open one of the files on my mac, but the workhorse is my PC--so, Im still stuck in a rut...

I had this same issue with a CS5.5 project - I swapped out the I-Frame Only MPEG with Microsoft AVI and everything worked fine after that.  Thanks.

>>This project contained a sequence that could not be opened.  No sequence preview preset file or codec could be associated with this sequence type.

I deactivated, killed everything with Adobe's name on it and restarted Premiere (which promptly failed to ask me to or provide any options for reactivation - Encore subsequently told me I was not activated - relaunching Premiere showed the 30day message - relaunching the Adobe Application Manager let me activaite.

That allowed me to open ALL the prproj's I was DENIED access to by Adobe's licensinig division..... Slap on the back of the hand for your new flangled licensing devs - I bet you tested the part that collects the money and blocks use of the program right?  Serious killjoy project manager...

And Priemere dev's - you should know better than to trust them when they say you won't need to update anythign for the new licensing  - update the damn eror rmesage: "Oh yea...sorry...I'm gonna go ahead and ask you to reactivate everything"

Reinstall?  What is this ATI?

Could a Disk Defrag cause this scenario?  I just came on here with the same issue as described at the top of the chain and the ONLY thing I have done between the time my project worked and the time that it didn't (with the aforementioned issue) was perform a Disk Defragmentation.