I tried it on Office XP (Windows) and it doesn't happen there. As I don't have an Apple, I can't compare with Office 2004.
The workaround is easy. Again, that's in Office XP, but you can easily try it in your Mac version:
- enter ends the number and goes to same column, next row
- arrow down does the same
- arrow right ends the number and goes to next column, same row
With the enter and arrow keys near the numeric keypad, this is easier and faster than switching between keyboard and mouse for each cell. It takes some getting used to, but you'll manage that.
There always are minor incompatibilities between versions. This could be one of them. If you complain with Microsoft they either can tell you why they did it, or schedule a correction for this bug for the first service pack. If you don't complain with Microsoft, they won't do anything, I'm afraid.
I am using Office 2008 on a Macintosh. I have noticed that when I enter a number with a negative sign and then click on the next cell to continue entering numbers, Excel acts as if I have created a formula and adds the cell reference to the negative number. This does not occur with positive numbers (i.e., numbers with no sign in front of the number). This behavior seems inconsistent. I have no clue why this is happening, but sometimes I can be clueless. I've checked all the preferences and searched the internet, but have found no reference to this specific issue. Am I having one of those clueless moments, or is something weird here? Your help is greatly appreciated.