You know what I got asked today in the Win7 webcast I did? If one had to install SBS 2008 in order to deploy Windows 7 and I honestly said no.
Win 7 http://msmvps.com/media/p/1696447.aspx
Vista – http://msmvps.com/media/p/1618969.aspx
In fact the two handouts I did for how to deploy Win7 and Vista are strictly about installing on a SBS 2003 not SBS 2008. Another question I got was whether you could go from 32 bit to 64 bit (you can’t — you have to clean install).
Can a Windows 7 machine and a SBS 2003 cohabitate nicely? Yes. Will Windows 2008 make the smb file transfer goodness faster? Honestly yes, but 7 will play nice with both server platforms.