I know we should use 'either' for negative sentences. But I heard native speakers said:
"I can't wait too." instead of "I can't wait either".
Is it okay?
Native speakers might say it, but it is incorrect. And it's also the kind of incorrect that doesn't have any redeeming qualities, i.e. it doesn't sound particularly good or appropriate for casual speech. This is unlike, for example, "me and Jeff went to the bar," which sounds great, despite being wrong. :)
If you are a non-native English speaker, please consider checking out my research survey, described elsewhere on the Using English forum.