I frequently update my personal wiki from work.
Seen through a traditional lens, this violates the standard corporate legal boilerplate. When signing the initial work contract (for both salaried folks and otherwise), employees have to sign a statement that says that the copyright for all of their work is owned by the company. (see: work for hire)
This has been challenged on a few fronts (especially regarding work that's done completely separate from work, on personal time, using personal computers — e.g. contributing to a completely separate open source project after-hours). However, it is generally held to apply when working during paid-time, when using company-owned computers / networks, or when working on something that is remotely related to something you're paid to do (even if you completely re-write something in your personal time, if it's a rewrite of something you wrote for your employer, it is nonetheless a derivative work).
Updating my personal wiki during paid time, using company computers, with content that is perhipherally related to work, would seem to be a clear copyright violation.
Historically, the line of demarcation between what you could take with you to your next job was: what you can keep inside your head. If there were skills that you learned on-the-job, then great, that was considered "job experience", and was owned by the employee. However, if any of that "job experience" resided on paper notes, then those notes had to stay with the original company when the employee moved on.
Increasingly, more and more of our personal knowledge base is being moved out of our brains and into computers. To maintain the old standard of "whatever you can keep inside your head" is to shift the balance-of-power towards the employer.
[1] see "transactive memory" on Wikipedia and elsewhere