It should replace it with the default font, which I think has to be Myriad. (you may be able to change that via your default files, but I'm not sure.) Myriad and Helvetica are kind of similar, so it might look the same to you. It shows you the font that is assigned as that will not change unless you change it. That way, you can open the file even if you don't have the font and not accidentally alter it. It should be showing it in brackets or italic (I forget...) to indicate that the font is missing. If it is not showing up as missing in the font list, there may be something wrong with your font cache, in which case you can clean it.
You will need get your customer to provide you with the fonts they used or an outlined version of the file.
Thanks, Jesseham. You're right, it's Myriad. I know that because if I place a duplicate of the text right beside it, and change THAT text to Myriad, it looks the same as my substituted text. It DOES show up missing in the font list (under Type>Font menu). I can understand, I guess, why it lists the original font in the character palette so, as you mentioned, we know what it was originally; however, I wish SOMEWHERE it would tell us what it actually IS.
(To complicate matters, a newbie in our department apparently opened the art, ignored the font message, left the substituted Myriad, created a PDF, and sent to the customer for approval, which they did. Which puts us in an awkward position, especially now that the artwork has "RUSH" status, if we now go back to them and say we don't have the font...)
I cleaned the font caches twice, via Fusion's font cache cleaning.
Does anyone out there know how to change the default substituted file? I'd like to have everyone's Mac substitute Courier or something equally obvious.
Note: I DID find a posting to change the default font for when you create a new document (http://forums.adobe.com/message/2976470#2976470
...but I'm not sure that addresses the problem of font substitution when you open existing art with a missing font. I'll keep researching.