When I import pushpins back into Microsoft Streets and Trips (after having exported to Excel and going over the header and 'information' lines with a fine-tooth comb for consistency), the pushpins then have three lines - the label, latitude / longitude, then the 'information' line.

Is it possible to import such that the latitude and longitude is used to place the pushpin but does NOT appear on the pushpin when the information bubble is displayed? (like how it appears when the pushpin is manually placed)?

Yes.

Go to the Properties of the Pushpin set, go to the Balloon tab, and uncheck the lines for the data you don't want to show.

That'll do nicely.