Is it possible that adding generated data to your data set will decrease the fraud/non fraud ration and make your dataset more representative / usable ?
At GenieLog, we are producing test data for designing and testing fraud detection tools. We our generator GEDIS Studio we can define regular profiles and fraudster profiles, instantiating each category to a customizable ratio (for ex. 2 % of customer will have fraudulent usage of generated events.)
We did it successfully for telecom CDR (http://www.gedis-studio.com/online-call-detail-records-cdr-generator.html) and Credit Card usages. There's a freely available access to the online generator on http://www.data-generator.com
I'm pretty sure that even if the tool is not matching your needs at least the approach can be valuable. Otherwise I would be interested to read any objection :)