I create a lot of WordPress templates but most of my work is done in Fireworks not Photoshop (http://www.iconzplanet.com/) for example. So I can include an unflattened PNG file but won’t have a PSD file to include.
Is this a big deal to people who are looking to buy WordPress themes? Will it greatly decrease my number of sales?
Absolutely! And you can always export your PNG to layered PSD