As a rule of thumb I always do everything on a router through a LAN connection because of this, but it won't lock you out completely [most likely] (I've done this on a WRTG model). Just use the generic log-in (admin, admin). If at all possible stay on a land line though... as this is a possible scenario (you could experience the dreaded "brick wall" syndrome because of improper flashing or faulty software).
If you're fortunate enough to have bought this recently... you can return it without saying anything about the flashing process (not always the case though).
Ya the DD-WRT firmware should go into defaults as far as I know. I know they have had some trouble in the past of not doing so, but it has been some time since then. Possibly they went into linux default (admin, root)?... but I can't be certain of that... as with any flashing of 3rd party software there is a certain amount of risk you take.