Here's what I want to ask you. How would your mother respond if you sent her an e-card instead of an actual card for Mother's Day? Forget the fact that my mom doesn't use a computer, if she did, and I sent her an e-card instead of actual card, I would never live it down.
I do send e-cards when I think the occassion and the recipient call for it. Susan and I send Hallmark Hoops and YoYo e-cards to each other all the time. Hoops and YoYo can brighten anyone's day.
But e-cards and e-vites set a very different tone than paper cards and paper invitations. Every once in a while, I pick up a little extra work, ghost writing web content. I've written many articles you'll never see my name on about greening weddings, and one of the things I always offer up as an option is sending out wedding e-vites, but I always add that sending an e-vite instead of a paper invitation will set a certain tone for your wedding. A very casual tone. Send a wedding e-vite, you're more likely to get people showing up in clothes from the Gap.
I'm all for making this world greener. But I think that people are most important. So, if it's very important to my mom that I buy her a paper card, then I'm buying her a paper card (and if I can find one made on recycled paper, that's a plus). For other things, like inviting our friends to our annual Christmas party, e-vites are very appropriate.
The environment is very important, but it's not everything. If you've got a hip, tree-hugging mom who would appreciate an e-card for Mother's Day, then go for it. If your mom treasures her cards, then give her a card. I don't think dishonoring your actual mother to honor Mother Earth is worth it.