I use SMB exclusively to stream locally stored media (ISO, MKV, and many other codec/container types) between my four ShareCenters (2 DNS-345 and 2 DNS-343) and Boxee Box, and can only presume the A400 GUI has an SMB connect option where you add network shares. The Boxee Box (and other media players I've previously owned) had options to add network shares via NFS, SMB, etc.
BTW - I am looking at the A400 as a replacement for my aging Boxee Box (BB). . . thoughts? Are you familiar enough with the BB to comment on how it stacks up to the A400? My primary use case is streaming locally stored movies/TV/mp3 files and personal videos with detailed XBMC files and box art. I rarely use online apps, and have no use for online streaming media services (e.g. Netflix, Hulu, etc.). What I appreciate most about the BB is the user friendly GUI, segregation of Movies and TV shows/series into dedicated sections, auto-discovery, and IMDB integration. I'm hoping the A400 expands on this level of media organization and takes things to the next level. From what little I've read about the A400 (including viewing YouTube walk-throughs), it appears the A400 will exceed these expectations.
Some of the BB's failings are a poor mp3 player, limited XBMC handling, inability to queue (or loop) multiple videos for playback, and lack of nested video searching. Also while Movies and TV series are handled well, organizing personal videos is meager at best.