Aaand, me again

This time I want to store a reference to the entry that I'm going to write.
For testing I whipped up some code that tries to store the ID of the entry simply in an extra field, something like this:

function saveCalEntry($data, $form) {
$entry = new simpleCalEntry();
$form->saveInto($entry);
$IDofthisdata = $entry->ID; // How do I get the ID of this new entry?
$entry->recurring = $IDofthisdata;
$entry->write();
}

But unfortunately it seems that ID isn't set before write(). Any ideas on how I could go for it?

Thanks in advance /Carsten