This is true, but it also creates enormous opportunity for potential suppliers of the component that apple has secured.
Apple's operating here to ensure they are able to meet their own internal demand. They have a lot of difficulty doing this, as was acknowledged in the last conference call. This means that they lose out on sales they would have otherwise made.
Over time, the component industry grows capacity to support Apple and everyone else. This does take time and is a competitive advantage for apple in the short term. But it is not sustainable. Meanwhile the long term pricing contracts on critical components is a financial (rather than supply) advantage.
But note that the barriers to entry are only for competitors creating a derivative product.
If HP or any of these other companies had spent the last 10 years working on a tablet design, they would have likely made different choices than Apple and likely arranged for a different spec components. It is only because they want to quickly ship a replication of the iPad that they are going after 10 inch displays.
If they'd developed their own touch UI and their own ergonomics they would have their own specs and thus be seeking different components.