If you want alerts to automatically pop up so to speak, when a task is due you may want to take a look at the demo of
Scriptfire, from Dacons - http://www.dacons.net/fmplugins/scriptfire/
Unlike the current version of filemaker, this lets you schedule mulitple scripts to run by times and dates, which are remembered even when closing and re-opening the solution. So for instance, after setting a follow up, you could run a script which shows you a message relating to this follow up and allows you to go directly to the record.
For some form of calendar / scheduling system, I would highly recommend taking a closer look at Seedcode's new calendar
They have just released their new free version and the pro version is due out on the 16th which is truly an amazing piece of work and easy to modify. I did a complete integration, with loads of modifications into our existing solution within a week (a video showing this is on their site)
You could combine the two fairly easily (its my intention to) and create a very powerful solution solution
Ive never used ACT, but it sounds like these may be what you are looking for.