I'm not sure that ISPs really care so much about it one way or another.  
I'm pretty sure mine has no specific policy -- or none that I've found.  
And I do know that an implementation of UDP multicasting I did was
tested on a system at work where we have a hardware firewall and
successfully resulted in those UDP packets getting to another system at
home that was listening for them with the app I designed at the other
end for that purpose.

It would be a good idea for the OP to visit Sun's tutorial at
<http://java.sun.com/docs/books/tutorial/networking/index.html>.  It has
reasonably good information on both TCP and UDP networking and addresses
multicasting.

= Steve =