Bertalan Mesko, MD, has compiled a nice summary of the conflict of interest (COI) issues that arise when employees of pharmaceutical companies edit Wikipedia pages. Interestingly, Mesko notes, "disclosure of COI is not required by any Wikipedia policy." That's different than COI policies elsewhere on the web.
According to Mesko, some pharmaceutical companies do, however, aspire for transparency in their edits:
Some editors have voluntarily chosen to disclose a conflict of interest by including their employers' names in their account names, e.g., all these folks from GlaxoSmithKline. More self-identify on their user pages.
And, for those who do not, he says:
Corporate IP addresses are highly traceable. An employee editing from the office should assume that s/he's hung a big sign on the edit saying "I'm part of a pharmaceutical conspiracy".
If you're looking for a quick primer on this issue, I recommend the entry.