Sometimes I see old questions or answers posted by users that now have high rep or even are diamond mods. Just out of curiosity, it would be nice to see (e.g. by hovering over the user name) the reputation this user had at the moment of posting.
While this would certainly be an interesting piece of information, before we can think about implementing something we must ask if it will be useful and beneficial.
To What End?
As we know (and have repeatedly discussed) reputation is not equivalent to programmatic skills, historical reputation even less so. Let's take a few examples.
Reputation is not gained by learning
An interesting, but certainly true, fact. Reputation is gained mostly from answers. This generally means that you are exactly as smart before writing a stellar answer as you are after it. Obviously, users gain knowledge and skills over time (usually over the course of doing their jobs) this, again, won't be reflected in their StackOverflow score unless they go in and answer more questions, which, as I mentioned usually indicates that they have more knowledge before they gain the rep.
Usability 101: Do not include irrelevant information
If we add a feature which presents users with information they don't need, they will attempt to factor it in anyway. We explicitly do not want this, because the new information (rep at post-time) is meaningless or (at worst) misleading.
The reputation graph on each user's network profile should cover your needs on this.
And there you have the reputation of the user at the time of the post.