Here's also a super cool script for fixing those seams that show up when you want to export. http://www.zbrushcentral.com/attachment.php?attachmentid=287372&d=1324640357
tip: if you're using the 2.5d canvas for tileable textures, which itself is fantastic, don't press 'bake' in the layer palette. Doing so will cause a seam within zbrush; not even the export script will be able to save you then.