You seem to have covered a couple of the possibilities I can think of without seeing the code...
1. A set of <p></p> tags is wrapping the text in that cell
2. The cell is aligned vertically and the first cell in the row has more text, forcing the cell to be larger vertically, causing the vertical alignment of the cell in question to leave space above.
3. You may have a broken table nested withing another table. Browsers have issues with broken table code and will display things differently depending on what you're using. DW is basically Safari, if you use something else to preview, it may just be seeing a problem Safari doesn't. That being said, on to 4 and 5...
4. There may be an error in your table code. Run your page through the validator here: http://validator.w3.org/
5. There may bean error in your css. Run your page through the validator here: http://jigsaw.w3.org/css-validator/
Without a glimpse of the code, That's all I can think of, off the top of my head.