Not buying photoshop
Photoshop was one of the Mac's killer apps before it came to Windows. Under Apple's guidance, it could have made the mac a much more successful platform. Apple has always focused on markets it already has a strong foothold in - graphic design is a major one.
Adobe has made photoshop bloated, change the UI with every major version, and don't have Apple's creative atmosphere. Now Photoshop on Windows is surpassing the Mac version (with things like GPU acceleration in CS4). This could be a major market to slip from Apple's hands.
Applications like Photoshop and MS Office have been showcases of how professional applications can be very profitable when ported the Mac. This has helped convince developers to recognise the OSX as a sensible development platform. Now though, Adobe is focusing on Windows, and leaving OSX behind. Apple should respond, and has only 3 options:
1. Buy Photoshop. Refocus it for the Mac. Would be expensive, so the Windows code will have to be kept to get that money back, and even that could take a while.
2. Compete with Photoshop. Bad idea. It doesn't have the photoshop brand, and releasing a competitor would likely kill off Mac Photoshop entirely, along with a lot of other Adobe Creative Suite applications. Potentially catastrophic.
3. Press Adobe to focus on the Mac. Would probably be the best solution. Jobs noted in 97 that, even though Photoshop was one of the biggest Mac applications, Apple didn't support Adobe enough. They never asked Adobe how they can make a Mac that ran PS better. If Apple provided engineering support to Adobe to help them leverage OSX, it could rebalance PS in favour of the Mac. The danger is that Adobe will want flash on the iPhone as part of the deal, which would anger both Microsoft and (more importantly) Google, and is something Apple really don't want happening anyway.