Samsung is a very big producer of flash memory. While that isn't unique, if Apple has to fill their large requirements from other suppliers, those suppliers may decide to charge more (or have to charge more given other demands on their production capacity) if they know Apple doesn't have Samsung as an option...
On the other hand Apple is the single biggest buyer of NAND Flash in the world, if Apple goes elsewhere, Samsung's foundry would find it really hard to fill the capacity.() IIRC, since Japan Tsunami everybody's been scrambling securing components but the demand was relatively soft, e.g. Qualcomm ends its recent quarter with 59 days worth of inventory. So now everything is cheap including NAND Flash, as a matter of fact Apple in its financial outlook for Q4(fiscal) stated a buyer's market for multiple key component will positively affect Apple's GM.
HTC's tablet is also under ITC's investigation, initiated by Apple.
Except the A5 SOC nothing Samsung makes for Apple is unique enough. And TSMC is actively testing mass producing Apple's IC.
BTW: Apple's utility model of essentially a giant iPod touch is ridiculously hilarious.