I was interested last week (this was in October 2005) to hear that the Lancet had published an editorial questioning how Reed Elsevier (the publisher of the Lancet) can be involved in organising arms fairs, and at the same time be a major health publisher. Reed Elsevier sent a PR person to the Today programme to argue that the arms trade is legitimate and it is better that someone organises things like this responsibly, and that each Elsevier journal was able to hold its own opinions. Perusal of the Lancet itself reveals that there was a letter signed by a dozen or so people, some signing on behalf of campaigning organisations, protesting that Reed Elsevier's involvement with the fair was incompatible with the Lancet's principles. I imagine the editorial was a response to the letter.
The editorial, the letter and a response from Reed Elsevier to that letter are all available via ScienceDirect
The BMJ published a news piece about all of this, also, at the time.
Updated bit: The BMJ today (16th March 2007) has an editorial about this whole issue: http://www.bmj.com/cgi/content/short/334/7593/547?etoc. There are links to campaign materials on this topic. The editorial argues that this is not the BMJ attacking a rival journal, and that the BMJ does not want to see the Lancet "diminished". Personally, I remember being struck in my first professional post, in a library with vast collections of historic medical journals, by the campaigning nature of the Lancet. I personally wonder how compatible this campaigning nature is with a publisher that organises arms fairs, however independent each journal is from the publisher that owns it.
Second updated bit: Guardian, 23rd March, has a report about the number of letters published in this week's Lancet about this.