I now have over 270 books on my Kindle (80% of them still to be read), and don't seem to be having any problems in adding to them. I monitor a couple of blogs that alert me to free books.
In fact I have unfortunately got into the habit of checking whether new titles that others talk about on their blogs are available on the Kindle. I'm seriously thinking I'll have to make impulse buying harder by taking the extra step of disconnecting my Kindle account from my credit card!
What seemed to happen frequently about a year ago, a barrier that said "not available to Australian subcribers", doesn't seem to come up so often now. I have a rule that I basically don't buy e-books if they are over $10, and many of them are much much less than that, sometimes even free.
Australian publishers are still not really with it yet, and tend not to supply e-book versions of new publications, but that will get better. Certainly many are willing to supply early reviewers with a pdf copy.
You can check the books that I have read on my Kindle through this label: kindle and last weekend I added a gadget in my side bar that lists the last 5 books I read on my Kindle.
I thought you might be interested in a presentation I gave in Melbourne on Saturday to teacher librarians on e-books and e-book readers.