Linds - I think that it's a tough position to be in. One can never assume that a parent or parents should pay for any part of the wedding. I know from experience, I didn't want to assume, although my parents helped, as did his, but until I had the dreaded conversation with my parents I wasn't banking on their support. Times are EXTREMELY different then years ago when the brides family paid.
I think the best way to handle this is to talk to your mom. Nothing will be resolved by assuming and not talking about it. Old fashinioned or not, she may surprise you. Just talk it out and make it point that you need to nail down your budget and you don't want to take anything for granted...