After upgrading to the latest version of Virtuemart (1.1.7) when i checked out i received the error:
'Please provide an amount and click Update Totals.'
The reason for the error is I had forgotten I'd previously installed Planet-B's patch to generate invoices after payment is confirmed by Paypal and not before.
When I upgraded Virtuemart it overwrote two files that had been altered by Planet-B's patch:
.....html/checkout.index.php
.....classes/ps_checkout.php
To fix this, download the patch and locate these two files. Then search for 'planet' and you will find the code snippet that you need to add to the updated versions of those files.
http://www.b-planet.com/VirtueMart/paypal-patch.html
Hope this proves useful to someone :)