@ James Woods - No Room For George's question, above, helps to determine the answer - Hannah also spoke of offering Samuel ("I will give him to the Lord all the days of his life"), and we know it was not a human sacrifice she was referring to.
As mentioned earlier, the mention of his daughter's virginity is likely pointless unless the idea that she would live as a virgin perpetually is being conveyed. The idea doesn't seem to be 'I'm going to die, but I have never had sex and never will'. The two just don't jibe.
It's extremely unlikely that any attempt to offer a human as an actual burnt sacrifice would have been tolerated in Israel at this time. It's highly doubtful that Jepthah could have pulled it off.