Well, I think I just about have this whole mapping in Excel thing down, so why not share the techniques with everyone else. Quick note that I'm doing these maps with the latest, greatest versions of Excel and Paint and nothing else.
The first step is setting up your grid. In general, this involves eyeballing the fields into squares, and then adding a border to all of those squares using whatever color you like. In the example below, I'm using a light blue.
Now, I color in all of those squares with the same color blue, and then cut out the passages and chambers by changing those squares to "no color", though I suppose coloring them white would work just as well.
At this stage, you can add in walls using thick lines (again, using the same color as above), doors (they're just small rectangles), stairs (see below, took me a while to get these right), pillars, statues, etc. The newer versions of excel also allow you to freestyle draw shapes, which are good for irregular pools. For pools, I do a tight, white dot pattern over the blue. For chambers that are going to be natural caverns, just get the overall shape right at this stage.
The secret door is just an "S" (Arial 12 pt.) in a text box with no outline and no background.
The stairs are a long trapezoid, no outline, with a pattern of vertical lines or horizontal lines, depending on the direction the stairs face. Yeah, I'm kinda proud of figuring that one out - I originally tried drawing in the lines, but could never get the spacing correct.
To make the pointed room, I added a couple right triangle shapes of the blue color. I then add room numbers using Arial Narrow, 9 point. You can also add outlines of rounded shapes over rooms, coloring in the bits outside the outline in the next step.
We now highlight our map, hit CONTROL-C to copy, and open up MS Paint. In Paint, we paste in the map. If we want to turn any of our passages or chambers into tunnels or caverns, we just use the paintbrush (same color as background) to draw in the natural walls.
And, lo and behold, we have a workable dungeon map. It's not perfect, and there are some limitations, but it's not bad for using a couple pretty basic programs. Whether this will work with the Open Office version of Excel, I don't know - I'd love to hear from somebody who tries it out.