Well Gartner reckon "by 2010, WSS will replace a majority of Windows-based, LAN file shares. (We expect explosive growth in WSS deployment and usage.)"
So if you're not rolling out Windows SharePoint Services to your customers (and remember – if they've got a Windows Server 2003 Server License and CAL they have the rights to use it) then who is?
I've always said WSS has got huge potential for partners – but also huge risk. It's a great way for another partner to get their foot in through the door with your customer.