[R] simple reshape

[Troels Ring]

Dear friends - this is a very simple question -  I have a data frame
'data.frame':   87 obs. of  3 variables:
  $ ID   : int  1 1 1 2 2 2 3 3 3 4 ...
  $ prep : num  1.18 1.38 1.34 1.93 2.38 2.24 1.17 1.13 1.21 1.89 ...
  $ postp: num  0.63 0.71 0.75 1.01 1.12 1.07 0.87 0.64 0.7 0.8 ...

- 29 persons (ID) each measured three times before and after an 
intervention: prep and postp -
I need data rearranged like

ID    time    val
1    1    prep
1    2    postp
1    1
1    2
1    1
1    2
I cannot make reshape or stack do the trick.

I'm on windows 7
R version 2.15.2 (2012-10-26)

Best wishes
Troels Ring, Nephrology
Aalborg, Denmark