A Clumsiness falls in the category of childhood "quirks," meaning normal variations in development that correct themselves - or at least become less quirky - in time. As you've found, it's often difficult for parents to tell the difference between normal, temporary toddler clumsiness and a developmental problem.
To get a better idea of whether you should be concerned, first ask yourself what your daughter's temperament is. Impulsive children tend to act before they think, running headlong into things and tripping over objects because they're always looking ahead at their target rather than where their feet are. Normal toddler clumsiness steadily improves as children become less impulsive and learn to think before they act. Also try keeping a diary, writing down the signs of clumsiness that concern you most and charting your daughter's progress over the next six months. If you see that the clumsiness does not improve, you should certainly mention this to your pediatrician at your child's 3-year checkup.
There are two other problems that could be causing the clumsiness - both require a bit of parental detective work. If your child has a vision problem, stumbling and bumping into things could likely result. Signs that she may not be seeing clearly include tilting her head and squinting (children do this in an attempt to bring blurred objects into focus); sitting increasingly close to the TV; crossed eyes; and frequent blinking. If you notice any of these signs, have her vision checked by a pediatric eye doctor. Other possible reasons for clumsiness are flat feet and toeing-in - which are normal in many toddlers, but if extreme, can cause them to trip. If you suspect her feet are to blame for the clumsiness, have your doctor check her while she's running. Tripping that stems from this common orthopedic cause nearly always resolves itself by 3 years of age.
Keep in mind that any neurological problem worrisome enough to cause clumsiness and tripping is usually apparent in other skills. A child your daughter's age (2 1/2) should, for example, be able to speak at least 50 words that you understand, follow simple directions ("Go find your shoes"), kick and throw a ball well, stack approximately six blocks, jump up and down, and pedal a tricycle. If your child seems right on track in other developmental milestones, it's unlikely that her clumsiness is due to a neurological problem.