Saving a spreadsheet to a new file (Save as... on the File menu) does not force 
a formula that contains CELL("filename") to be evaluated freshly. To the best of 
my knowledge, there are three ways to get evaluated but all three involve a 
manual process:

1) CTRL+SHIFT+F9
2) ALT+F L
3) Code the formula with something variable concatenated with the function call 
{(e.g. T(A1)&CELL("filename")} and change that variable item after the Save as.
..

I believe this is caused by the fact that there is nothing variable in the 
format of the CELL("filename") function call i.e. "filename" is a constant 
string. Thus there is nothing to force a fresh evaluation of the formula.

It appears to me that what is needed is a flag associated with the formulae that 
identifies that a formula contains this particular construct exists and when 
Save as... is invoked formulae should be checked for this flag.

Here is how you can perform a test of this problem:

1) Create a new spreadsheet.
2) In cell A1 enter =CELL("filename")
3) Select Save as... from the file menu and give the file a name.
4) Look at A1 and you will see that its value is unchanged.
5) Either perform CTRL+SHIFT+F9 or select Reload from the file menu to see that 
this value should indeed have changed.

For a designer of a spreadsheet application to have to tell the "great unwashed" 
user community that they need to perform one of these extra steps is not 
satisfactory.

-- 
Jim