When I first started using Numbers '09, I thought this was still the case -- because that's all the arrow keys seem to do in this version, too. But here's the secret solution, which isn't documented in the User Manual nor in the Keyboard Shortcuts list available in Help: Hold down the Option key before pressing the arrow keys (this won't work in Numbers '08). With the Option key down, you can then move around the active table and select cells using just the keyboard. Release the Option key, and the last-highlighted cell is inserted into the formula.
After I found this, I couldn't believe it wasn't documented, so I dug a bit further. It turns out it is documented, but only in the new (and very well done but not well publicized) Formulas and Functions User Guide. You can find this guide via an entry in the Help menu in Numbers '09, which takes you to this web page. There you'll find the Formulas and Functions User Guide available for download (or just click here to download [4.9MB]). Read on for some more keyboard function entry tricks from the user guide...
This guide explains every one of Numbers functions, and offers a number of useful tips. After stumbling on my Option key trick, I found a whole section devoted (on page 27) to using the keyboard when entering functions. For instance, to refer to a range of cells (instead of a single cell), select the first cell in the range using the Option key trick, then add Shift to the Option key, and move with the arrow keys. Release both Shift and Option when the range is set.
To refer to cells in another table on the same (or another) sheet, select the table by pressing Command-Option-Page Down (or -Page Up). Once the table you'd like to use is selected, keep the Option key down, but release the Command key, then use the Option or Shift-Option methods to select a cell or range of cells.
To set absolute and relative attributes on a cell reference, first insert the reference into the function, then use the arrow keys (without holding Option down) to position the cursor right next to the reference. Now press Command-K to cycle between the various relative and absolute addressing options.

