Please look at the BES connector for Google Apps feature chart:
http://www.postini.com/webdocs/gapps...ture_chart.pdf
Tasks are not synced
Memos are not synced
No wireless backup
So you can only have those things local to your BB device and it will be always out of sync.
So as much as we all like the cloud, it is not a full Exchange (or Lotus Notes or Groupwise) replacement yet.