The problem is that when I take the CellEdit's ReadOnly seriously there may be a conflict with the grid which maybe can be allowed for editing. And what if only one particular cell would allowed for editing which for all the others it would be disallowed?
This is a very confusing situation (ok - the current situation is even more confusing...).
The readonly properties are determined by the state the worksheet protection as described in the wiki. Look at attached demo file. Here, the entire worksheet is blocked from editing except for the yellow one, you can verify this with spready or the demo_ctrls sample project. So, the basic idea is to leave the ReadOnly property alone and to define protection using the properties of the worksheet from which the visual controls get their data and their behaviour.