Hi,

I found the following navigation in spreadsheets:

Set focus to first cell of sheet  cmd+Home
Set focus to last cell of sheet  cmd+End 

Set focus to a cell one page up  Page up
Set focus to a cell one page down Page down

Set focus to first cell in row  Home
Set focus to last cell in row  End

Set focus to first USED cell in col  cmd+Arrow up
Set focus to last USED cell in col  cmd+Arrow down

This is a mixture of Windows and Apple style.
In Mac OS Home, End, Page up, Page down is equivalent to moving the scrollers, so they only scroll and 
don't change the focus!

I didn't found a exact guideline for navigation in spreadsheets (OSXHIGuidelines.pdf) and even Apples 
iWork suite is not a good paragon in using there own standards! In the sheets application Numbers 
there is NO navigation except moving from cell to cell by Arrow Keys. That's poor.

I would suggest the following as the closest Mac-way:

View first cell of sheet   Home  (like use the scrollers) 
View last cell of sheet   End   (like use the scrollers) 

View a cell one page up     Page up  (like use the scrollers) 
View a cell one page down    Page down  (like use the scrollers) 

Set focus to first USED cell in row  cmd+Arrow left
Set focus to last USED cell in row  cmd+Arrow right

Set focus to first USED cell in col  cmd+Arrow up
Set focus to last USED cell in col  cmd+Arrow down

Set focus to first USED cell of sheet  -not defined- (it is easy to use cmd+Arrow up and cmd+Arrow left)
Set focus to last USED cell of sheet  -not defined- (it is easy to use cmd+Arrow down and cmd+Arrow 
right)

And let me say: great work is done from the Aqua team! But to find the right functions on the right keys 
is very important for fast working.