Double spaced, IE 6 sp1

IE6 bugs? oh, that's a good belly-laugh!
IE has never been known for its compliance to the standards, if that's what you mean.
You'll get 'double spaced' text because its interpreted as a paragraph end. Holding shift down generally forces a simple line break.

As its 'by design', there isn't.
You'll find, of course, that any web page text separated by <p> tags has lines between. Using <br/> starts a new line, but you're still in the same paragraph.
Whatever way Excel does its html markup, you'll likely find that its something of the above.

If you want the carriage return to be NOT equal to a paragraph mark, then do the <br/> tag (Control-Enter, I think?) (or use a Text Area entry box, if you really don't want HTML tagging)

If it IS ok to be equiv to a paragraph, but you just hate the HTML default of a paragraph putting a full line space between paragraphs, like I do, then change the style on your HTML.

put in a new style like this, in the HTML header, or in an included style sheet (this gives half a line space above and below):

<style text="text/css">
p {
margin-top: .5em;
margin-bottom: .5em;
}
</style>