As far as I know it's up to the stewards whether or not to let them in, this will depend on things like Practice times. If somebody who regularly qualifies for the race has a mechanical failure, I'm sure they will be able to race.
This is interesting. Surely this is open to abuse and could bear some intersting results.
The last time they used the 107% rule, if memory serves me correct, teams had access to spare cars, which means that even if a driver crashes his car in the practise session, he can then use the spare car in qualifying. Failing that he has the option to use his team-mates car to qualify.
Now, with the banning of using spare cars (without taking some form of horrendous penalty), the repercussions of pushing too hard in an earlier session and crashing out (just as Alonso did at Monaco) before the qualifying session starts, could have a major effect on the outcome of the races.
I can't say that I am a fan of this 107% rule. It certainly makes a new team joining F1, less likely as any new team has to consider the possibility that they may never ever start a single race during the course of the season, due to the 107% rule. As we saw this year, when you are new team, it is VERY difficult to compete at the same level as the established teams.
Sponsors are also likely to have clauses in their payment terms, such that if a team fails to start a race (perhaps due to the 107% rule), sponsorship money may be withheld. This will hit a new team VERY hard.