@travelplanet24 had the idea to ask users to come up with a catchy tweet that would be retweeted many times for marketing purposes. They were sucessful enough, since the majority of the greeks that I follow retweeted #tp24Lon almost making it impossible to read the stream that I follow.
I felt DDoSed and I objected strongly. I will not go into legal, technical and moral issues of what can be considered spam in a medium like twitter. I am simply one of those 45,7% users who use the web browser as their client and such campaigns, when successful, are annoying because of the limited (== none) filtering capabilities available from the web client. Unfollowing people to prevent this is just like disconnecting from the Net to stop a spam outburst. To be fair, of the people I follow only one more complained and a second one retweeted my objection.
What I find more interesting is that @travelplanet24 effectively managed to construct a retweet botnet where, in contrast to the usual botnets, the participants consciously retweeted the message. While the first time that such a trick is used may be considered successful, it may be a one time only stunt. With 45,7% of the users still using the web client, imagine two or more different such campaigns running at the same time. We need both a better web client and better standalone clients.
At least @travelplanet24 said that their next campaign, if any, will last less time.
Managing organized complexity 
I cannot do anything about the web client but we can do something about the desktop clients. The next major version of firestatus (http://code.google.com/p/firestatus) will start having some basic features that other twitter clients already have.
To the essence of your post now. I think that a key here is the “consciously” in “the participants consciously retweeted the message”. The difference between mail and twitter is that in mail you need to have control of what comes in your inbox and this control is compromised by spam. However in twitter, by design you cannot control your stream (unless you start unfollowing people). There are unsolicited e-mail messages but there are not unsolicited tweets. When you follow someone you accept that whatever she/he says will appear in your stream. That’s twitter. Think that even your dialog with travelplanet might be considered uninteresting and annoying by your followers.
“Think that even your dialog with travelplanet might be considered uninteresting and annoying by your followers.”
Not exactly the same. For my followers to find uninteresting my dialog with @travelplanet24 (or anybody else for that matter) that would mean that they are following both me and the other end of the conversation (and in fact I am not even following them, yet I can converse with them). I do not have the graphs handy to estimate the overlap of followers.
That ‘s even worse. If your followers do not follow travelplanet24, they will only see your part of the conversation and this will definitely be annoying and boring. But again, that ‘s twitter. If we do this conversation on twitter, some of our co-followers might find it boring and follower that do not overlap will definitely find it annoying. Are we spammers to them ? I don’t think so.
Depending on the settings on the web client they might or might not see halves of conversations. They do see them on FriendFeed though. The easy way out from your question is “spam is anything I think it is spam”. That is why I avoided discussing what is twitter spam.
As I said, I felt DDoSed. I carefully choose who to follow and suddenly my stream was full of a certain tag from a certain marketing campaign. The fact that people consciously behaved like a botnet annoyed me the most. The human botnet differs from the traditional one where there exists a central command and control. Despite of what annoys me, you or anybody else, the formation of such a retweet machine is worth noting, because twitter just like every application on the Internet must adhere to the robustness principle:
Be conservative in what you sent; be liberal in what you accept.
and not only in protocol dialogs, but in actual data also. And in this case, the clever marketeer did not actually send any data.