I have a worksheet that has a formula in all cells in row H that says
if there is a comment in row G then put the current date/time in
H(current cell). My formula looks like the following in H5;

=IF(ISBLANK(G5),"",NOW())

The problem is that if I enter a comment in any cell in row G it
updates all of my dates in row H.

Using Excel 2007

I use Excel 2003, but 2007 might be similar.

One way is to use a circular reference. To allow circular references, use
Tools Options Calculation
and check the "Iterations" checkbox.

Then, try something like this in H5:
=IF(G5="","",IF(H5="",NOW(),H5))

Adjust to suit.

Caution: allowing circular references can be a risk. Later, if somebody
adds a circular reference in the workbook by mistake, Excel won't flag it
as an error.