I'd like the donation list to display a little more info for anonymous donors, and in particular display something like "Anon. donor" instead of an empty space.

For unregistered users, the paypal IPN returns their name anyway, so it is possible to retreive that as well.

I have modified function lm_paypal_donations() to do these two things:
1. if the donor is unregistered but the name is recorded in the transaction, display the transaction name (first_name & last_name)
2. If the donor's transaction is empty (say if you manually added the donation or it came from somewhere else) then display "Anonymous donor".

To use this modification, replace the entire while loop starting at line 550 with this snippet:

  while ($don = db_fetch_object($result)) {
	if (!$don->name) {
		if ($don->txn_id) {
			$sql = 'SELECT  d.first_name,  d.last_name FROM {lm_paypal_ipns} d WHERE d.txn_id="'.$don->txn_id.'"';
			$name_q = db_query($sql); 
			while ($names = db_fetch_object($name_q)) { 
				$name=$names->first_name.' '.$names->last_name;
			}
			if (!$name)
				$name=check_plain("Anonymous supporter");
		} else 
			$name=check_plain("Anonymous supporter");
	} else
       $name= l(check_plain($don->name), "user/$sub->uid");
	$rows[] = array('data' =>
      array(
        $name,
        check_plain($don->item_name),
        format_date($don->datepaid, 'small'),
        check_plain($don->mc_gross),
        check_plain($don->mc_currency),
      ),
    );
  }