It's not clear to me whether or not I'll be able to use iCloud with my current Apple ID from my old .Mac / MobileMe days... and furthermore, since I don't have access to the iTunes betas, it's not clear if this question is even answerable yet.
You'll probably best to wait until iCloud is out of beta to get the final answer on this. To many things need to occur before apple goes live with iCloud so what may work now, may not in the future and vice versa
Your Apple ID remains active even after your MobileMe subscription expires, so there shouldn't be any issue. Do you use it for iTunes or any other Apple services? You can make sure it's still active by logging into any other Apple service with it.
Your Apple ID remains active even after your MobileMe subscription expires, so there shouldn't be any issue. Do you use it for iTunes or any other Apple services? You can make sure it's still active by logging into any other Apple service with it.