question regarding large international organisations adoption of SPF

Hi All,

I'm really new to SPF and it sounds great but I have some questions 
regarding it adoption in large international companies.  Basically I 
work for a company and we have a product that allows clients to send 
emails to their customers (All opt-in) and not SPAM.  My question is 
with regard to my companies clients where they are large international 
organisations where there might be 2 MX records for receiving mail, but 
maybe 10 - 20 out bound servers spread across the globe plus dialup 
users which will use which ever mail server they can send through based 
on the ISP their using in the country their in at that time.   
Implementation of SPF for some of these large companies could cause big 
problems for their remote/dialup users.  And could cost hugh sums of 
money for them to comply with and use SPF fully without problems.  Is 
anyone on this list using SPF or thinking about using in large 
international companies? Maybe I'm not getting my head around this 
properly?
Gabe