Others have given solutions with extensions in gnu sed or tricky perl commands. There's however a very easy way to solve this with the unix toolkit that should work everywhere. No need for perl or awk or sed or anything. Suppose the input file is named inp. Then run the following shell commands.

( cat inp; echo ) > gre
rm spl.*
split -l 25 gre spl.
head -qn-1 spl.* > out
rm gre spl.*
[download]

The output is in the file out.

Update 2010-03-07: See Re^2: Joining two files on common field for a list of other nodes where unix textutils is suggested.