There are a few threads about this but i think it is quite a problem.
It seems that any newlines in the description field (in catalog_product_entity_text) is run through nl2br before being displayed.
This makes sense if the field is used to enter plain text but does not make sense if entering html. Seeing as the field does accept html then i think there is a problem.
When using a WYSIWYG editor it becomes even more absurd. The editor will already add <br /> for linebreaks (or wrap in <p>) and the displayed description will add another <br /> for each ascii line break (\n).
Someone has suggested using http://www.textfixer.com/tools/remove-line-breaks.php to remove all line breaks but my client can hardly be expected to use that on the source after adding or changing anything with WYSIWYG editor.
I have imported my products automatically with the html from our old site so either i need a fix (instructions to overload so as to not modify the core and keep my mod when there are updates would be great) or a way to strip all newlines from that field in the database (a long way from ideal as it doesn’t cater for future product entry)