Hi,
For the mashup, what post type are you hoping to display? One common ‘gotcha’ is that by default, WordPress only checks post type ‘post’ and the first page’s worth of entries.
It is also possible you’re running out of memory if you have a large blog. If so, you can limit the posts to be selected, by category, post type, etc.
Check out the documentation for some examples if you need a different post type or want to check > 1 page of posts for map locations.
As for Google Earth – no, it’s a separate API.