require(ggplot2)

## Loading required package: ggplot2

require(grid)

## Loading required package: grid

p <- 
  ggplot(data.frame(a=c(1,1),b=c(1,1),c=c(1,2)), aes(a, b)) +
  geom_point() +
  facet_wrap(~ c) +
  theme(strip.text.x=element_text(lineheight=4, color='red'))

element_grob.element_custom <- function(element, label="", ...)  {
  g1 <- textGrob(label, ...)
  height <- max(grobHeight(g1), element$height)
  gTree(children=gList(g1), height=height, cl = "custom_strip")
}

# give a height
grobHeight.custom_strip = heightDetails.custom_strip = function(x, ...) 
  x$height

# silly wrapper to fool ggplot2's slightly paranoid checks...
strip_custom <- function(..., height=unit(1, "line")){
  structure(
    list(...,height=height), # this ... information is not used, btw
    class = c("element_custom","element_blank", "element") # inheritance test workaround
  ) 

}

p + theme(strip.text.x = strip_custom(height=unit(3,"line")))