I was chasing some internet links around the other day and stumbled upon A123's Developer Kit:
If I am reading this correctly you get 6 battery cells, each at 3.3v (for a total of 19.8v) for $129. For $220 you can double that to 12 cells to get 39.6v. There isn't much information more than that but it would appear that it would be an interesting option for small applications.
I did a search for it here but couldn't find anything so forgive me if this has been posted and is otherwise old news.
I'm new to this but doesn't this seem like a reasonable alternative (even at a cost) to a deep cell lead acid battery for smaller applications?
EDIT: Here is the link to the spec sheet: