It should be noted that Brokeback Mountain didn't just release in 5 cities. It released in 5 theaters. That's it. Just 5. Given the targeted marketing and the size of the markets, I don't anticipate it doing well in wide release. I'm not sure that it will.
Syriana had a very comparable initial outing. It will gross about $25 million when all is said in done. It's market is much, much, much less confined to LA-SF-NY. Same goes for "Punch Drunk Love" which eventually made $17 million.
I'd compare it even more to "Melinda and Melinda," which went on to make less than $4 million.
Drudge compares it to other "Live Action" releases. Which is silly, because live action movies have never used such a release schedule, except for strictly micromarket ("art house") releases. Only Disney has ever done such releases. Compare Bareback Mountin' to Pocahontas, which made $450,000 per theater on six screens, the Lion King, which made $793,000, or even the spectuacular failure of Anastasia, which made $120,000.
But that's normal for a release of this kind. We can deny it all we want, but the critical acclaim is going to make this a decent box office performance. The important thing is that that doesn't really matter much.