I don't know that I would consider it lying. I've told her what Im doing in ways that she understands. Like if Im sending important emails or something, I will tell her that Im doing my school work. She relates the computer to me doing school work. That way, I don't have to explain it to her. I've done stuff like that when I'm having my time and can't or don't want to play, but she's okay with it. It's far and few when it happens. LOL
I sell things over the internet and so my son knows that most of the time when I am using the computer that I am working. Sometimes though, like when I just need time to think, or what ever I will tell him I am working so that I dont have to play, and be loud. I feel bad for doing it but he knows that when mommy works on the computer, that we get our play money. Meaning the money I get is for us to go out and have fun.
On days that are particularly busy and I'm just too tired to play with my kids, yeah - I'll tell them I'm busy. But unless I really don't have the energy to play with my kids, I'll get down on the floor and start playing with them - nothing beats seeing my kids smile and hearing them laugh, and knowing that I'm the one that made them do that!