The current work around for this is to either disable UAC or to run both Outlook and Tasktop/Eclipse as an administrator (e.g. Find Outlook.exe, right click to open the Properties, go to the Compatibility tab and check off the box that says "Run as Administrator"). If you change the settings to run as an administrator, make sure that both Tasktop and Outlook have the same settings otherwise they may not be able to communicate.
With Office 2007 and higher, you can disable the Outlook warning messages via the "Trust Center". You an access the Trust Center in Outlook via Tools > Trust Center and selecting the "Programatic Access" tab on the left. Select "Never warn me about suspicious activity" or "Warn me about suspicious activity when my antivirus software is inactive or out-of-date" if Outlook detects that you have valid antivirus software installed.
The ugly details: This problem results when incompatible authorization levels are used to execute Tasktop and Outlook. For example, Tasktop is now launched by default with administrative rights (to avoid other UAC related problems such as registering a COM dll for communication with Outlook), Outlook on the other hand is by default launched with the current user account level privileges. When this occurs, Vista blocks Tasktop from communicating with a application of lesser privileges (in this case Outlook). This case happens when Outlook is opened before Tasktop. If Tasktop is opened first and causes the execution of the Outlook.exe process, Outlook will run with the same privileges as Tasktop and therefore will synchronize properly. One negative side effect of this scenario is that if the user subsequently tries to open Outlook (as a regular privileged application) it will fail to launch (even/only with Outlook.exe having already been launched by Tasktop).