Speaking from my experience, in which I'm in charge of a development team in one of those huge companies from the corporate world, and I run technical interviews every now and then, you don't have to worry too much about it...
As long as you are using a framework that was not 'invented' by your company (in case it's a small dev shop, not freaking google), you are in good hands in terms of using an open source framework. Jumping from Yii, to CodeIgniter , to Zend is not that hard, and when I make technical interviews i always take into consideration if they use any frameworks in their daily work, that's right: 'any'.
On the other hand, someone who comes to me looking for a job and it's not using any framework, that's a -1 right there, just like it's a -1 for someone to say they love so much their framework they wouldn't consider something else, or even worse, to work with bareboned-PHP.
Just like dukeofgaming mentions, at least you'll know some concepts like ORM, OOP, etc that will come handy in other frameworks and even other programming languages.
However, i always ask possible candidates if they know why their are using that specific framework, ask yourself that or to the senior members of your team, someone must have taken that decision and they probably have a compelling reason to do so (or perhaps the decision was taken after a few drinks at the local pub...).
As long as you stay out of hacks, hybrid implementations (half done with Yii and half with barebone PHP, with bits of CodeIgniter) you are in good hands learning Yii... Just remember there are other things out there and keep the learning curve steep, because at some point or another, Yii won't be enough.