found a nice blog about reversing and android cracking by Dan, who was kind enough to point out that the well-known site crackmes.de has some android crackmes (just search android).
his blog also introduced me to a tool called dex2jar that will sort of convert a classes.dex file into .java source. of course, it's not perfect and it's still new but it can be useful for getting a quick overview if the dalvik is too confusing.
here's Dan's article on android reversing: http://www.kizhakkinan.com/?p=39, check it out. it's got pictures!