I think we're converging on a good syntax, but I don't think the choice of nothingness to represent an open range is a good idea, both because it will probably create grammar conflicts now or later and also because it actually is sort of confusing and unintuitive to read given the rest of our syntax. I suggest using UNBOUNDED instead.
As much as it reminds me of the window syntax I loathe (ROWS BETWEEN UNBOUNDED ....gah), I'm inclined to agree with Robert here.
It also probably helps for code forensics in the sense that it's easier to text search for a something than a nothing.