No, Adobe and Microsoft do NOT rule the eBook roost. 
also: the sites you mention are not, imHo, the best sites for buyin eBooks. try
Fictionwise.com and i think you'll find that the more consumer-friendly
secure and insecure formats from PDM and Mobipocket are broadly represented. alas, the decision of pricing and format is up to the publisher, so YMMV depending on the title you want.
as for the site you bought from, i second the suggestion: write customer service and make an assertive,
persistent case that you bought the ebook with the understanding that the new Adobe Reader would display the eBook on your PPC. ask for an alternate format, or a refund. if they won't budge, you could talk to your credit card company, if you used a CC. misleading compatibility information on the vendor site ("Adobe eBook format" ... "Adobe files can be read on PPC") could be considered sufficient for a charge back attempt. :evil:
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ebooks.com, for instance, seems to have an inordinate number of Adobe titles. goody for them. but you have to really read their FAQ stuff to find the details:
http://www.ebooks.com/help/tools/readerstable.smm
http://www.ebooks.com/article/78.smm