I dropped 5pm an email to ask about their API. Here's their response:
To allow a truly two-way communication between your current products and our service, an open API should be defined.
We plan to work on the 5pm API sometimes in the near future. This will enable other applications to load/save data to 5pm. I'm not familiar with Sage products, but if it has its own API, the same can be done from 5pm - accessing Sage API to load/save data.
Definately an up-and-coming product to watch then, as it seems to be getting some good reviews online.