There's a function in the package splitstackshape called cSplit which is perfectly suited for this task. Simply pass ";" as the separator and "long" as the direction to get what we need.
> library(splitstackshape)
> dat <- data.frame(V1 = c("x", "y", "z"), V2 = c("b;c;d", "d;ef", "d;ef"), V3 = 1:3, stringsAsFactors = FALSE)
> cSplit(dat, "V2", sep = ";", direction = "long")
# V1 V2 V3
# 1: x b 1
# 2: x c 1
# 3: x d 1
# 4: y d 2
# 5: y ef 2
# 6: z d 3
# 7: z ef 3
