I'm not quite sure why this is happening, but for some bizarre reason, the second column in the first row of a table always has the text beginning as if there's a blank line. I've checked the code to see if something was inserted by accident, but there is no line break or anything else between the td tag and the text in my table cell. I've double-checked the alignment and the cell's vertical alignment is set to "top."
In the editor, everything looks normal. But once I switch to live view or upload the file to the web, the blank line appears in the table's second cell.
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.
I seem to have found the problem. In the Design view in the Properties box, one column had the Format set to Paragraph and the other had it set to None. Unifying this across all of them looks to have fixed the issue.