A seemingly interesting HP15C emulator for the PocketPC appears in:
http://microsoft.handango.com/PlatformProductDetail.jsp?siteId=75&jid=BF2C84CABF6633A5832424CAAEE298C3&catalog=0§ionId=0&productType=2&platformId=2&productId=49174
I have not used it, this is just a pointer for who may be interested...