Ok, so I'm trying to get rid of my car (though I won't be completly car free because the wife will have one, though she is a bike rider as well and keeps the use down). Here is my one biggest obstacle. I commute most days to work, it's a 40 mile roundtrip with a lot of those steep/long nasty Austin hills. I am also an avid mountain biker who only has a singlespeed mountain bike. By the time I get home from my commute I'm to tired to change and switch bikes. I don't want to ride my mtb to work, that would take forever and wear out my tires way to fast. I have a Novara Randonee that I commute with, what I'd like is some ideas to get to work with my mountain bike, that way I can stop on my way home and ride some local trails. I have a modified Burly trailer for my groceries, but I don't want to pull it 20 miles, it's not that good of a trailer. I also don't want to just strap the bike to my back and carry it for that long. What I'd like is some way to take the front wheel off and put it on a tray and strap the front wheel to the bike.
I don't want to buy an xtracycle (well, I do, but don't want to spend the money on it), so this project needs to be done on the cheap (around $150 bucks).
Ideas?

A 40 mile round trip every day is an unusually long bike commute, and if you add the burden of somehow ferrying your mountain bike with you it is hard to see how it will be manageable. If you don't have the energy to go mountain bike riding at the end of your commute, how will you find the energy to complete the trip home after towing your mountain bike to work and partway back and then trail riding - that's even harder.

What about leaving a cheap mountain bike locked at the trailhead? You're planning to leave your Randonee there while you ride the trails, so presumably you think it's a safe place to lock up a bike.

It's more a matter of being to lazy to change clothes once I get home in the A/C. I've done the commute plenty. It's not that bad, really. I'm not going to ride a cheap mountain bike on these trails, they're way to difficult and would break the bike (or me). I wouldn't hesitate to lock up the bike.

Are there any places close to the trail head that you can leave/lock up your mtn bike? A friend's house, a bike shop, a train station with bike lockers?

If not, maybe you could get a second set of mtn bike wheels, with slick/durable tires on them, and ride those to work. On the way, lock up your off-road wheels at the trail head. On the way home, ride to the trail head, switch wheels, and go?

Or, how about just having the mtn bike loaded in your wife's car, ready to go as soon as you get home, so you don't even have to stop in the house. If your wife rides, too, she could meet you.

...I commute most days to work, it's a 40 mile roundtrip with a lot of those steep/long nasty Austin hills...
Your commute will put you in great shape, that's for sure.

In winter here the sun sets as early as 5:30 PM. You might want to keep the car as an option till you're sure riding in the dark won't be a problem.

Sometimes you can get a bus boost. All the Capital Metro buses have bike racks and the monthly bus passes right now are incredibly cheap.

I've don't use electric assist myself but if I had a daily 40 mile commute with hills I'd certainly consider it. Not just for the hills, but also to accelerate across problem areas like major intersections or bridges without shoulders.

If I had that Novara, I would never consider commuting with a mere mountain bike!

I think you need some practical solutions to winter, bad weather, etc that goes along with bike commuting. At 40 miles a day, there will undoubtedly be days when you would need a rest... that's when it's nice to put a backup plan into action. If you want to keep car use down, you would need to be comfortable with bus routes, as Platy suggests, or some other method of getting to work. You should make sure to investigate this before the weather turns nasty.

I'm not a serious mountain biker, but I am a real fan of using a mountain bike for commuting.... with proper tires and handlebars. A tire like michelin country rock (google it) is relatively flat resistant due to its fairly thick layer of rubber. Which is why i love that style of tire for commuting. But they are also fat enough to handle some pretty serious offroading. You will not have the traction of knobbies though, that's for sure.

Another option would be to take spare tires (or spare wheels&tires) along with you and switch.

Taking 2 bikes for the whole long commute is probably never gonna be a good idea.

And I agree that if it's day-in, day-out, all weather, you might not have energy to do much riding at all outside of your 40 mile round trip.

Might be expensive, but get a cyclocross bike and just do both on it. Specialized Tri-Cross for instance. Fast as your Randonee on the road, but will also handle some trail riding too.

I think you should probably take it slow. What if you commute most days on the Randonee, and then maybe two times a week on the Mtn bike, and hit the trails those two days. I get tired of commuting on the same bike each day anyway, so I switch up.