I work p/t from home about 25 hours a week, primarily while my kids are napping or after they've gone to bed for the night. My job is pretty simple (making calls to the clients of my employer to ensure that they are pleased with our service), and the pay isn't great, but it's enough to allow me to stay home. I never wanted to be a SAHM until my first DS was about 6 weeks old, and I realized I didn't want to be away from him for 40+ hours a week. The very thought made me cry. So when he was about 5 months old (and I had been working f/t for about 2 months), I found my current position, and I've been here for almost 3 years.
Since I work from home, I don't have any work friends (except the ones I still keep in touch with from my f/t job), but I've made lots of friends through our local MOMS Club and MOPS. I really prefer MOMS Club because it's very kid-centered; other than MOMS Night Out once a month, we don't have any activities at which kids aren't welcome. Many of the moms I know through both of these clubs (probably about 30-40%) work part time, so it's not at all unusual around here. Maybe it's the area we live in ...it's suburban, but definitely middle class; many of us need that second income to do our kid-centered activities!
When I joined MOMS Club 2 years ago, I was very hesitant to mention that I worked p/t. But it is SO not an issue now. Actually, I'm the president of the club now, and 2 of our other board members also work (one works full time!). As a matter of fact, I've noticed that our most active members - the ones who attend the most events and are always willing to help out - are the ones that also work.
As for the comment about "I like to use my brain so I could never be a SAHM," well, I don't like to debate - and I've been on both sides of this issue, I promise - but rest assured that we SAHMs (yes, I consider myself a SAHM even though I work p/t) don't let our brains go idle while we're at home. I don't know any SAHM who sits around eating bonbons and watching Oprah.
For those of you who feel like you don't fit in either place, I definitely encourage you to keep looking for your niche - there are mommies out there in your situation, I promise! I've done the library story times, Kindermusik, Gymboree, rec center classes, etc., and all are great places to find other people to hang out with. At this time in my life, though I do have interests outside of my kids, it's easier to hang out with other moms because they understand why I need to go to McDonald's instead of a "real" restaurant, why my activities need to end each day by 2 (for nap time), and why I sometimes have to interrupt the conversation to break up an argument over a dump truck.