Hi Ken,
It is 'cause the way guid is formatted from CRM. Try this for final :
<Button Icon="/_imgs/ico_18_debug.gif" JavaScript="var url = getSelected('crmGrid'); url = url.toString().substr(1, 36); window.open('http://blah.net:84/Accounts/' + url + '/Documents%20for%20Review/Forms/AllItems.aspx')" WinParams="" WinMode="2">
Regards,
Chinmay