Right. Typically you submit an Intent To Use when you have a mark that is registerable -- I don't have a registerable mark without the evidence of secondary meaning.
So I suppose the answer to my original question is really in the details of what qualifies as evidence of secondary meaning, (outside of simply being in commerce for 5 yrs). If anyone has info on this, please pass along!
There are some common misperceptions about what trademark registration does for an owner.
Registration gets you nice perqs that enhance your rights and make it easier to win in court down the road, but it neither creates the trademark, nor ensures that the mark will not be challenged, nor ensures that secondary meaning will be or has been found.
As Jeff and others have pointed out, in the US trademark rights are created by use. And first in time is first in right. So an intent to use (ITU) filing is just a place-holder that lets you get "dibs" on the mark going back to the date of filing instead of the actual date of first use. You still have to use the mark before registration issues. And the issue of registrability will be the same as in a regular application.
So, what's a business to do?
Before investing in branding, what you want to do is get competent trademark counsel to have a thorough trademark clearance done -- by a qualified search company who knows what to look for and covering both registered marks and "common law" (unregistered) marks -- and then have counsel review the results and advise on the likelihood of problems arising. (Including whether secondary meaning will be an issue.) Then, if the results aren't good, consider that it may be best to forego a nifty name that's iffy, rather than investing in it and waiting to see if a lawsuit ruins it down the road.
And realize that an opinion letter may "hedge" in both directions: it won't guarantee that the mark is absolutely good to go, since no one can do this, and it likely won't say "do not use the mark, it likely infringes" to you either. Many attorneys believe it wrong to make a negative opinion too strong, since a bull-headed client who goes ahead and uses a mark against advice of counsel could end up having the opinion letter come back to bite the client later in litigation -- not that the client wouldn't deserve it! So take the hint if an opinion letter is discouraging without absolutely forbidding the mark you want.
As for proving secondary meaning, apart from length of time and there being a lack of evidence of confusion, if you need to give affirmative evidence of public recognition, you can do this by a properly constructed survey -- done by someone who knows what they're doing: asking the right questions, in the right way, of the right sort of sample of consumers.