Don't know if it'll ever happen to be honest, but I'd think it is just what the Glazers would want, and not because they're American and familiar with it in the NFL, but because it would make life easier for them when it comes to paying off the debt. For example:
If there were to be a salary cap, and United's total wage-bill (£42.7m according to results back in March) was automatically cut by 50%, then to the Glazers, that would be like a whole new revenue stream in itself, as it would mean tens of millions of pounds (over £20m) from what used to be wages, spared, making it a bit easier for them to pay back the banks/hedges. And if a big name or two hands in a transfer request, that would give Glazer and his spawns the excuse/ammunition they need, making it a bit slightly easier for them to flog a player or two to the likes of Real Madrid, Barcelona, Milan, Juventus, Bayern, etc for as high a fee as possible.
Personally, I wouldn't mind a salary cap, and see many of the overpaid players in top flight English football brought back down to earth, even if it does mean some of best talents in this country leaving for Spain/Italy, let them show their true colours. At least we might be able to identify those who are in it for the shirts rather than the money, and if they don't like the cap they can piss off elsewhere to a country where a cap doesn't exist, I don't care if they are top quality or not. It would also help United's long-term future (as much as I hate the three cunts) when it comes to paying back those debts.