Here is a paper on the results of interactions with external matter en-route: http://arxiv.org/abs/1202.5708

With a couple of popularizations:

http://www.theregister.co.uk/2012/03/04/killer_warp_drive/

http://www.universetoday.com/93882/warp-drives-may-come-with...

(Essentially, particles encountered en-route pile up near the boundary and are released in a large burst when you turn the thing off, which is potentially very bad for the local environment, of which you are a part.)

And here's a paper on the horizon-like properties of the warp bubble boundary: http://staff.science.nus.edu.sg/~scilooe/srp_2003/sci_paper/...