Maybe the way to start with liability is to start by holding the users of software liable for bad things that happens as a result of software they bought fucking up. This will hike up the insurance costs of the users in the case where they have software that disclaims all liability because the insurer can't recoup its costs by suing the software supplier.

This increased cost to the user results in them being more likely to pay the increased costs for warranted software. The software house uses this extra money to take out liability insurance (the total cost of which will be somewhat less than the total cost of the increased premiums of all the customers who used the unwarranted versions), bites the bullet of warranting their software and works to reduce the risk by improving the quality of their software.

This could be seen to be an inhibitor on the takeup of open source software, but in fact it creates a market opportunity for the likes of Cygnus/Redhat/Whoever who is prepared to fill the market requirement for warranted software by selling warranted support contracts for open source software.