How to get a vertical geom_vline to an x-axis of class date?

Even though I found Hadley's post in the google group on POSIXct and geom_vline, I could not get it done. I have a time series from and would like to draw a vertical line for years 1998, 2005 and 2010 for example. I tried with ggplot and qplot syntax, but still I either see no vertical line at all or the vertical line is drawn at the very first vertical grid and the whole series is shifted somewhat strangely to the right.

gg <- ggplot(data=mydata,aes(y=somevalues,x=datefield,color=category)) +
      layer(geom="line")
gg + geom_vline(xintercept=mydata$datefield[120],linetype=4)
# returns just the time series plot I had before, 
# interestingly the legend contains dotted vertical lines

My date field has format "1993-07-01" and is of class Date.