I will soon try to replace the tooltips on cell's in the following manner – it may or may not work, but if it does work you could use the same technique …
1) make a custom formatter for the column, which will insert a hidden field with a known class name into each cell
2) once the grid is loaded, i'll search for all these hidden fields ( $(.clsHiddenField)) and iterate through them grabbing they're parent (ie. the cell)
3) set the cell's title attribute to whatever i like (you could set other attributes such as background colour etc.)
If anyone reading this has a nicer solution to setting the cell's tooltip please let me know