transactions not rolled back if followed by a variable_set

This is an interesting bug, and I'm not sure what is causing it. I suspect it is something in variable_set (like clearing the caches) which causes this problem. Anyway, it is easy to repeat and is consistently repeatable for me:

1. Install PressFlow Transaction
2. Create a new page node that has a php input format with the following body:

// Begin transaction
$txn = new pressflow_transaction();
db_query("INSERT INTO {variable} (name, value) values ('a', 'b')");
$txn->rollback();
variable_set('something', 'value');

3. Preview the node

Expected results:
Run the query: select * from variable where name = 'a'; should return an empty set

Actual results:
The record is inserted and returns a value when that select is run.