I upgraded bitcoind to 0.8 from 0.7, may I delete the old blocks? Upgrading to 0.8 from 0.7 I noticed the old blk*.dat files were untouched, while it was created a new directory called blocks/.

I guess I can delete the old blk*.dat files? Is there something else I'm supposed to delete?
Yes, if you don't plan on switching back to the old version you can delete the old blk*.dat files.

The final 0.8 release will include this little script to tidy your data directory:
https://github.com/bitcoin/bitcoin/blob/master/contrib/tidy_datadir.sh