Looking into flying Continental from San Jose, CR to Minneapolis with layover in Houston. The flight we prefer only gives us 1 hour, 24 minutes layover. Is that enough time to clear customs and immigration and make our flight?

For US citizens, I'll say it's enough. Fairly tight, but chances are good you can do it.

I agree. If you can, try and avoid a mid to late afternoon transfer when several flights from Europe dump hundreds of people into the system. But even then, your chances are pretty good. CO has desk just outside customs to recheck your bag and its a pretty smooth operation.

IF you have only carry on it should be no problem. If you have checked bags, then as rkkwan notes it will be tight but doable. We often have a similar connection returning from Mexico and make it OK, though once my bag did not make the connection and arrived n the next flight (they delivered it to my door.)
If you do have checked bags, as you exit customs there are CO staff right there to check your bag. You will need to go through security again.

Thanks! We will be anxious to get home to our kids at that point, and hate the idea of taking the later flight. CO offers a decent fare right now with good times.

Is there anything worth seeing in Houston that would make a 12-hour layover preferable to a 90-minute layover? I'm not about to book anything at the moment, but looking at some possible itineraries, I can see where this might be a choice.

In other words, is there a tour bus (like GrayLine or something) or some other choice readily available from the airport where it would be interesting to see Houston in a relatively short period of time?

There are things to see and do in Houston, but you have to rent a car and drive yourself.

Looks like the Space Center, for one. Grayline has a tour listed that appears to be cached on Google, but not on their current web site.

If you are at all into museums, the Museum of Fine Arts is excellent. Also in the museum district (oddly enough) are the Contemporary Art Museum , the Rothko chapel, and the Cy Twombley collection at the Menil. You could also grab a good meal in he general area (great moules frites at Montrose cafe on Westheimer) As rkk notes, you would have to rent a car and drive yourself, or fork over some major bucks for taxis. Allow at least an hour each way if you do drive, including rental car pickup and return.

Also in the museum district is the Museum of Natural History which usually has excellent exhibits and the Cockrell Butterfly House (a 3 story greenhouse where butterflies are raised - pretty amazing place). Both are within walking distance of Museum of Fine Arts, etc.

If you just want to do the museums, you can do it very cheaply. There's the Bus #102 which costs $1 each way. Then the Metrorail also costs just $1 each way.

So, $4 can get you down to the musuem district and back.

The 102 runs about every 20-30 minutes, and takes about 75 minutes, with a grand tour of an area with the nickname "Gunspoint".

Is the bus generally a crime-free zone? I was afraid to even ask about such a possibility, but maybe I have the wrong impression of Houston.

The bus is "generally" safe, especially for routes like 102 with the commuter coaches. But I did have someone try to grab my iPod once on a local bus.

BTW, "Gunspoint" got its nickname at least 15 years ago. It's not particularly dangerous these days.