There are two different edit modes in Gnumeric:

1. select a cell, start typing
-> left arrow selects another cell

2. select a cell, press F2
-> left arrow moves the cursor left

I'm asking that even if you start editing by using scenario 1, you could still
enter "F2 mode" by pressing F2.

Here's an example:
1. Select B3
2. Type "testig"
3. Press F2 (to enter "F2 mode")
4. Left arrow (don't go to A3)
5. Type "n" (B3 is now "testing")

Okay, silly example, but I'd like to always be able to enter the F2-edit mode to
use the cursor keys "normally".

I don't, however, think that F2 should toggle the mode. That would be confusing.
But if someone presses F2 while editing a cell, this is probably what he expects.