Grid of multiple ggplot2 plots which have been made in a for loop

I would be inclined to agree with Richie, but if you want to arrange them yourself:

library(gridExtra)
library(ggplot2)
p <- list()
for(i in 1:4){
  p[[i]] <- qplot(1:10,10:1,main=i)
}
do.call(grid.arrange,p)

take a look at the examples at the end of ?arrangeGrob for ways to eliminate the for loop altogether:

plots = lapply(1:5, function(.x) qplot(1:10,rnorm(10),main=paste("plot",.x)))
require(gridExtra)
do.call(grid.arrange,  plots)