Jane,
You raise some very interesting -- and common -- questions about homeschooling. We went through many of those exact questions when we were making the decision of whether or not to send our daughter (now 7) to public school, private or homeschool.
You also articulated the questions in very provacative ways. My wife is working on an ebook right now on the topic, those questions fit nicely into the interviews she'll conduct over the next couple of weeks with a number of experienced HS'ers.
I'm slammed with projects right now. But, I'll try to post a bit now... and then more later.
Let me say a couple of very brief things about it... based on experience:
* We originally investigated homeschooling because of my travel schedule -- as you know I speak at many conferences, corp meetings, etc. Up through pre-K my wife and daughter traveled with me 99% of the time. We get to visit and stay in some of the most wonderful places -- didn't want to give that up. (those are now used as "educational opportunities with our daughter) The time together -- that's why we married -- and then had a daughter in the first place -- was a primary concern. So that started us down the path.
* Socialization was our #1 "concern." Yet, the more we investigated, we realized that would not be an issue. Example: My daughter...
> Goes to a HS "co-op" on Mondays where approx 30 families rent a church sunday school building and have 4 class periods of varying subjects.
> Tennis lessons on Mondays and Wednesday
> Dance: Technique on Tuesday... Ballet, Tap & Jazz on Thurs
> Girl Scouts every other Tues night
> Wednesday afternoons she takes gymnastics -- in advanced level
> Fall and Spring she plays soccer with HS group (I take off Friday afternoons and coach in this group btw)
> One Monday each month is "Homeschool Skate" at a local skating rink
> One Monday each month is "Homeschool Bowling" at a local bowling alley
> She's in a HS girls club
> She's in a HS History co-op that meets monthly for "History Activities" -- each year, they cover a time period
> Our local HS Association has over 300 families and 1000 children... a massive resource with more activities than I can mention here (for all age groups)
NOTE: My daughter, in December, just hosted her "Karlista's 3rd Annual Girls Holiday Tea Party and Gift Exchange. Like 20 something girls attended. She consistently has 25 to 30 girls at her March birthday parties.
The child has NO social problems... she has a better social life than her parents

Has her own computer, email addres, IM account and cell phone.
* Education: The "If you don't know it, how can you teach it" argument is valid. However, there are SO MANY ways to overcome that. There are internet-based HS programs, co-op, tutors, university-monitored programs, etc. etc. It's absolutely overwhelming how many options exist. My daughter -- at 7 -- is one of the best educated, most well-behaved, socially developed children you will ever meet. And she gets to spend A LOT of time with us... her parents. That -- to me -- is priceless. We get to raise her, not the school system.
* Many people don't realize that they can teach their children MORE than the public school is teaching them (academically) in virtually the same time that they spend "Helping them with their homework."
* When HS'ers reach highschool, they can take courses at the local junior college, or community college, and get simultaneous credit. Take college-level math, etc -- while still in highschool. That's a huge benefit.
* HS'ers consistently place high in entrance / placement examinations for colleges. I'm telling you... these are really bright children.
Again, your concerns are VERY VALID. They should be thought out by anyone considering HS (not that you are).
It requires a paradigm shift...
It starts with motivation -- "Why might you want to do this?"
Then comes belief -- "Can you do it?" and "How?"
I can't wait until my wife completes her ebook -- I'll bring it to the board for reviews and feedback.