If you're like me and you have your reasons for not using Entourage, but you still appreciate its interoperability between email messages and todo's, I've found a few Applescripts you might be interested in.
First up is Mail2iCalToDo. Once a message in Mail is selected, invoking this script will simply take the message's sender and subject and create a todo in iCal. If you'd like to chose a default calendar for these todo's to be created in, run the script yourself by double-clicking it, without a Mail message selected, and you'll be prompted to chose a calendar.
Next is Fuhgeddaboutit, which builds on Mail2iCalToDo's capabilities by presenting a dialog through the todo creation process allowing you to chose a calendar, set a due date and append an event summary with each todo you'll create from a selected Mail message.
Last but not least is a set of Labels scripts. There is a separate script in this pack for each label color Mail can assign to a message (including one for no color), so these effectively allow you to assign colored labels to Mail messages on the fly.
I'd like to also mention Mail2iCal, from the same author of Mail2iCalToDo. Mail2iCal creates an event (not a todo), but I can't get it to work under 10.4.2. The script winds up trying to start Classic, which I don't even have installed. If there are any Applescript gurus out there who could help clear this up, I'm sure we'd all appreciate it.
Nevertheless, these scripts, combined with some kind of an application launcher utility such as Quicksilver, will let you get more done in less time, adding a nice boost to your productivity. Just don't tell your boss.