The primary advantage of ePub support for me would be that I could check out library e-books with it. Plus in the future as ePub becomes (hopefully) well-established, I'll be able to buy books for it with DRM that is standard across vendors enough to expect long term support for the DRM mechanism - i.e. I have a better chance of not rudely finding out that I can't read my content on new devices in the future. Right now, if you buy BBeB e-books or Kindle e-books for example, you could wake up one day and find that no new devices are still made that support your DRM'd e-books. You're then left either reading on an old device or losing access to your books.
Btw, one of the great things about the Sony Reader in terms of life expectancy is that you can load rtf e-books without any software via the SD card. Plus there are even great 3rd party programs to do format conversions to BBeB (you can find a bunch of them for free right here at MobileRead by some amazing developers). That means that if Sony support for the device ever falters, I can still continue to use it so long as the hardware and battery hold out for me!
Unfortunately, I don't really think that this is the sort of topic that will bring an avalanche of responses. I almost didn't even post to this thread even though I REALLY would like to see a 500 firmware upgrade. It's more the sort of thing that you just appreciate when you discover it has actually been made available to you. Let's hope that this is the case.
And if Sony does this, even if you don't own one, but are just considering a future purchase of a Sony Reader, surely it's one more indicator (along with things like the original design decisions to use an SD card reader and to publish BBeB format information and to support RTF, and to provide firmware upgrades in general, and to now support non-Sony e-book stores, etc etc) that Sony really will make the customer glad that he or she bought a Sony.