Yep. It has happened. More times than I can count on one hand. Moms and future Mother-n-laws who give more than 2 cents. More like $1.99. I kind of get why, and it almost...almost makes sense. As long as you've been dreaming of your wedding day- it goes double for them. So yeah, they want it perfect too. The problems usually arise when what you think is perfect, is most certainly not what she thinks is perfect. What's the best way to handle it? Play it it fair and be upfront from the get go. Invite the help where you do want it, and request where you don't want it. Really, you'll be so happy you did.
Oh, and when you ask your mom for help, make it special. Take her to lunch, talk it all out, and give her a nice gift. Emily Post just put out this new book, Emily Post's Wedding Planner for Moms. I think your mom and/or future mother-n-law will be tickled pink. Oh, and if you really want to impress her, show her that you got yourself the brides version. Even I use it- it does have answers to some tricky questions.