Using CiviCRM 2.1.0 stable with Joomla 1.5.7.
The problem is that when payment for a pending contribution is entered via the backed, no 'Transaction ID' field is offered. Here's the process to replicate the bug:
1. I register a contact for an event using the 'Event Tab' under the contact's record. I enter the 'Participant Status' as 'pending' and leave the 'Record Payment' option unchecked. the URL I'm using to register the contact for the event is: http://joomla.demo.civicrm.org/administrator/index2.php?option=com_civicrm&task=civicrm/contact/view/participant&reset=1&action=add&cid=105&context=participant.
2. Later (after payment has been confirmed) I come back and edit the event registration (under the contact's 'Event Tab') so that I can 'Record Payment'. I check the 'Record Payment Option, but when I do this there is nowhere to enter the Transaction ID.
This happens on the demo site as well (as you probably guessed from the URLs above).