No, ePub is ePub (unless it's the B&N flavour of ePub, that is
). You'll be fine buying from anywhere else.
We know that Adobe will be offering a new version of ADE (Adobe Digital Editions - the software that's installed on reading devices to read ePub and PDF files) which will support the new B&N DRM mechanism (in addition to the current one) from early next year. What we don't
know is how long it will then take device manufacturers such as Sony to produce new firmware versions that incorporate it. To say that Sony haven't been "quick of the mark" to take advantage of new ADE versions would be being over-generous to them.