Child Safety: Baby Walkers
No, it is not safe to use a baby walker. In a recent article in Pediatrics, it was estimated that there were 197,200 injuries to children younger than 15 months in baby walkers from 1990 through 2001.
As for walkers and physical development, you can find studies that suggest "improved" development and studies that suggest that they are detrimental to development.
Children developed quite well before the invention of walkers and the child safety issue is so paramount that I am strongly against them. Infant exercise saucers, however, are a reasonable alternative.