"It is the goal of MacRuby to enable
the creation of full-fledged Mac OS X
applications which do not sacrifice
performance in order to enjoy the
benefits of using Ruby."
- the MacRuby README
MacRuby isn't a 'translation layer' as Rob says. It's Ruby on the same object system that Cocoa is using. You can certainly build "first-class" applications with it, and also accomplish things that are inconvenient with Objective-C.
Be careful not to confuse MacRuby with RubyCocoa. Apple did not 'pull all the templates' for MacRuby, because they've never shipped by default. MacRuby is pre-release software, and Apple certainly isn't 'deemphasizing' it. I'd ask those that say otherwise to throughly research the topic, and ensure that they're making accurate statements.
Furthermore, LLVM's integration with Apple's platforms grows with each release. The next release of XCode will rely on LLVM for advanced code-completion, checking, and compilation. If Apple is deemphasizing anything it's the GCC.
One might also note that MacRuby has similar limitations in API coverage as Objective-C does: for instance, creating authenticated apps or accessing the keychain requires wrapper classes for both languages. It's safe to say a huge-scale application on the Mac will require the use of pieces of multiple languages for years to come, but MacRuby has a strong future and a usable, capable present. I'd encourage you to search GitHub for awesome MacRuby wrappers.
All the evidence currently suggests that you and I both can look forward to building apps of all kinds with MacRuby across Apple's platforms.