Back in the SharePoint 2007 days, I wrote about a workaround for modifying date formats in SharePoint Designer workflows. In SharePoint Designer 2010, when performing a lookup for a date value, you have some built-in formatting options with the Return field as list:
If the formatting option you need is not in this list, you can use the String Builder to format the date. This forum shows an example of this kind of formatting.
Also note that in SharePoint Designer 2010 workflows, there are built-in formatting options for user fields. If you do not want user fields to output as DOMAINuser (for example, 3SHARPdavidg), you can use the Return field as list to modify the format:
I am not terribly surprised that this functionality exists in SharePoint Designer 2010, as string formatting was a pain point in the previous release. I am, however, in amazement that it took nearly three years for me to find this in the product and subsequently blog about it.