Just say to client..Your site looks like this on a mobile phone..
test i phone.com and it could look like this...using mobile galore or wp touch.
I think with wp touch and the plugin keeping existing domains would be easier than mobi galore site and .mobi change.
I guess it would matter about the complexity of a site.. but most offline sites should be simple landing page with contact info.
non aff links below
WPtouch 2.0 Pro - The mobile theme solution for WordPress - optimized for iPhone, iPod Touch, Blackberry Storm, Android and More » BraveNewCode Inc.
Mobile Website ! Best mobile websites ( mobile web ) build easily with our mobi site builder.
I should give credit to Maria Gudelis. She mentions this method in a video with wp touch pro.