Quite simply, SPS going on its own isn't really that big of a deal. Motorola is such a vast company that it operates as several independent entities, SPS among them. The biggest potential challenge that an independent SPS will face is that of reduced R&D funding, as SPS will no longer enjoy the bankroll of an aggregate Motorola. A tighter budget will lead to either increased efficiency, the lack of which has stifled Motorola PowerPC efforts since the late Nineties, or oblivion. After starting with a bang, it didn't take Motorola long to stall out with PowerPC.
After IBM's PowerPC 601, Big Blue and Motorola collaborated on the Somerset designs. Among these processors was the venerable PowerPC 603, a low-cost, low-power part targeted at low-end and portable systems. The 603 flourished in the embedded and desktop worlds and acted as the cornerstone for virtually every one of Motorola PowerPC designs after it. It was also the engine behind Apple's first PowerPC PowerBooks and their consumer desktops.
Following up their success with the Somerset designs, IBM and Motorola created the PowerPC 750 — the G3 — from the core of the PowerPC 603. At the time, it was the cheap, fast, and simple processor that Apple required to weather a trying transition period. Soon after the G3, however, IBM and Motorola parted ways, with IBM leaving Motorola the Somerset facility. Sadly, Motorola was dependent upon IBM for architectural innovation, so this move was the beginning of the long, slow end for Motorola's PowerPC.
Not long after the Somerset split, Motorola began an old bad habit of rehashing ad nauseum, extending the 603 core year after year with add-ons, hacks, and kludges. This philosophy of upgrades translates into less performance and an increase in complexity with each new iteration, a recent example being the 500 MHz Fiasco, where Motorola's PowerPC G4 (another 603 derivative) was stuck at 500 MHz for 18 months. Apple was forced to downgrade their new Power Mac G4 processors shortly after introduction and weathered a horrific year and a half standing still while Intel and AMD raced ahead with faster and faster chips.
These problems, alongside a dodgy fabrication process, poor yields, and constantly slipping schedules, caused Apple to move away from its dependence on Motorola and the G4. With Motorola's development efforts pouring into the embedded market with the PowerPC 8500 line — yet another 603 variant — it became clear not only to Apple but to Mac fans as well that moving the Mac platform ahead in speed and performance meant moving away from Motorola as a chip supplier.
In the end, SPS having a go on its own won't really effect Apple as they move toward using IBM's G3 and G5 chips everywhere, especially as IBM's 750 line gains AlitiVec in its next major revision. Motorola has made this last move with SPS far too late to do Apple any good, though it became obvious long ago that retaining Apple as a customer was no longer a significant goal.