Just wanted to share what I noticed this morning when I was at the National Zoo to see Tai Shan. I had tickets for 1100 and got in line early. There is also a separate standby line to get in. Around 1050 they let just about everyone in the standby line into the panda house to see Tai Shan. The official viewing times are from 1100-1300 daily, so they basically started 10 min. early to let a group of standbys in. They also count the number of people who show up for each time slot and let standbys in if not everyone shows. So if you don't have a ticket and you show up around 1030 (for days like today, may be need to get there earlier during the holidays) I'd say your chances are very good to get in.

Tai Shan had just gone into a side den right before I got in and wasn't visible. After a couple minutes the keepers brought him out.