[BioC] splitting a data frame

[kfbargad at ehu.es]

Dear list,

sorry for this very basic question but I have searched the R manual 
without success. What function do I need to randomly split a 
data.frame into two data.frames (for example a data.frame containing 
200 rows and 5 columns into two of say 150 and 50 rows each)? Should I 
first create a vector of 150 random numbers between 1 and 200 (but 
how?) and then use subset() function?

Thanks in advance for your help
David