Well, VMware doesn't comment on future releases. Although everyone expects new announcements at VMworld in August, nobody actually can tell you whether there will be another 5.x version or 6.x or something else. Anyway, in the past one needed to attend the required classroom training only once and there was always a period of time - after a new version/exam was released - in which you could take the exam for this new version without attending another training. Also remember that the certification/exam is usually released a few moth after a new version.
With that said, I think you won't do anything wrong with going for the VCP5 at this time.
André