My thoughts are that milestones as a rule don't mean much, but there are some 'signs' that seem to be more common among the gifted, though clearly is not definitive. For example, alertness from the get-go, and some early physical milestones seem to pop up a lot. My first, 3yo, had incredible head control from the beginning - he rolled over average time (3-ish months?) but everything else was early, crawled at 5.5 months, walked at 8.5 months...of course, he didn't talk till after he was 2! My newest, 3 months old, is also exceptionally alert and had the same amazing neck control. he started rolling over at 3 weeks and desperately wants to crawl...and gets clearly frustrated that he can't do much more than move sideways in teeny tiny increments. (I laughed at one of the previous comments about 'swimming'...my son moves around a bit on his back by 'backstroking' as DH and I like to call it!)
It's hard not to compare the 2nd to the first, I'll tell you that...we keep saying, 'he could be average, but that's ok, we'll love him all the same'. still, we can't help but wonder what he will become. (I wonder if this is a full post somewhere...how NOT to compare your child went at least one is gifted. I'll have to look around.)