Fortunately it was noticed before we racked up too much of a bill (although have already run up double the bill we typically would for August and it's only the 20th!), so we took snapshots of the instances before terminating them - we then launched regular instances using the snapshots.
Made me wonder how many people have been caught out and will get a August bill full of max-priced hatred from Amazon. The problem seems to be peoples approach to spot instances - typically cheap, so lets set the max price to $5.00 (because the systems aren't throw away, aka not suitable for running as a spot instances) and profit. In theory, and practise, that does work (it worked for us for nearly a year) but just a few bad months would undo all that.
I tried to explain this before, that when it comes to using spot instances for platforms that aren't suited to it (and therefore setting the max price high to keep it running) that the house, Amazon, always wins.