Fighter player character attacks with a grenade-like weapons, e.g. Acidic Fire [ddi], can they still
Mark the target(s)?
From PHB1, pg76:
Note that there are no restrictions on this. (Hybrid fighters are slightly different and require the use of a Fighter class power to mark a target.)
If the grenade-like weapon requires an attack roll (and all the ones I've seen do), then yes, the fighter can mark the target(s).