If it's a business site, I think they should have a link that implies, in some way, that the site can be browsed with hand-held devices since a lot of business oriented people use handlheld devices nowadays. This would be more beneficial than saying the site is XHTML valid for a business site since it's unlikely that a business oriented person could link the two (i.e. that XHTML valid sites are also easily browsable with hand held devices)