ma (9/7/2009)
Now that's interesting. I filled "t" with your script. Including my initial data, there are 10288 rows. I ran the four queries and still get the only table scan on #4. This is on SS2008 Developer Ed.
Did you rebuild the index?
What I found was:
Initial conditions:
1) Index Scan
4) Table Scan
After adding 10001 rows:
1) Index Scan
4) Index Seek & RowID lookup
After rebuilding index
1) Table Scan
4) Index Seek & RowID lookup