A couple of years ago I was talking to a couple of friends of mine who were doing some work at eBay. It's always interesting to hear about the techniques people use on high volume sites, but perhaps one of the most interesting tidbits was that eBay does not use database transactions.
My immediate follow-up to the news of transactionless was to ask what the consequences were for the application programmer, in particular the overall feeling about transactionlessness. The reply was that it was odd at first, but ended up not being a big deal - much less of a problem than you might think. You have to pay attention to the order of your commits, getting the more important ones in first. At each commit you have to check that it succeeded and decide what to do if it fails.
I've been wondering about the usefulness of transactions for sometime and ACID in particular. One of the easiest to dismiss, suprisingly perhaps, is isolation. For example, skip list implementations ruin it as the data may change during iteration and it can easily be avoided by writing the information of one transaction to a file to be read by something else in another (either by accident or on purpose). The alternative might be something like CAS and RDF triples. Expecting application programmers to do it sounds awful however.
Also, noticed by, "Living Without Database Transactions".