If you attempt to create a capped collection with a maximum number of documents greater than 2^31 - 1, Mongo silently lowers this to exactly 2^31-1:

> db.runCommand({create: "cappedWithMax", capped: true, size: 10000000, max: 4294967296})
{ "ok" : 1 }
> db.cappedWithMax.stats().max
2147483647

If you leave the "max" field out of the creation command, you still get 2^31 - 1 as max:

> db.runCommand({create: "cappedWithoutMax", capped: true, size: 10000000})
{ "ok" : 1 }
> db.cappedWithoutMax.stats().max
2147483647

This seems to be due to this section of _userCreateNS in pdfile.cpp (at line 259 on current master):

        if( options["capped"].trueValue() ) {
            newCapped = true;
            BSONElement e = options.getField("max");
            if ( e.isNumber() ) {
                mx = e.numberInt();
            }
        }

I would expect that the range of valid values for max should be either a long (2^63 - 1), unsigned long (2 ^ 64 - 1); or alternately that not specifying max leaves the number of documents unbounded.