I have used similar techniques to achieve this in both dimensions, ie x+y.
The basic premise was ala google maps, a view port div, a massive div (map) within the view port for the scrollbar support, the concept of a virtual buffer area around the viewport and then a collection of tiles (tables) that would have data rendered into them and then be appropriately positioned into the buffer area around the viewport as one scrolls. Works well, have tested up to 40mm rows and columns and works in ie8 up to 4mm rows. Also supports random column and row sizes littered throughout the sheet. This solution does also break native in browser search though as mentioned in other comments.
The basic premise was ala google maps, a view port div, a massive div (map) within the view port for the scrollbar support, the concept of a virtual buffer area around the viewport and then a collection of tiles (tables) that would have data rendered into them and then be appropriately positioned into the buffer area around the viewport as one scrolls. Works well, have tested up to 40mm rows and columns and works in ie8 up to 4mm rows. Also supports random column and row sizes littered throughout the sheet. This solution does also break native in browser search though as mentioned in other comments.