Wifi – How to monitor the users’ actions on the network

User authentification for Wifi use 802.1x protocol.
To connect devices need a WPA supplicant such as SecureW2
Depending of the supplicant you use you will or not will be able to use do a SSO with the windows domain login/password.
iPhone and iPod touch have built in WPA supplicant. I don't know for PSP/BB. SecureW2 has a Windows Mobile version.

I'm sure that you could enable a captive portal for WiFi only without having to create to IP Network. You just need to put wireless access in a vlan and wired access in another vlan then put the portal between both vlan. This is like a transparent firewall.

802.1x need to have a supplicant on computers. If computers that need to use the Wifi are known you just have to setup the supplicant on them and it's a great solution. If you want to make your wireless access accessible by visitor or things like that it could be a nightmare because they need the supplicant etc..

A captive portal is a bit less secure and need user to authenticate manually each time they connect. It can be a bit borring.

A good solution from my point of view is too have both. A 802.1x access that give you the same as if you were wired on the lan and a captive portal that give you access to less things (access to internet port 80, limited access to local lan, ...)

You could setup a monitoring service such as Nagios.

You would still be relying on ping response to tell if they are up or not (unless you had an alternate way of checking such as an SNMP request to the VPN server/router).

However, you wouldn't be running it by hand, it would be automated and would give you a record of when the pings failed and when they were successful.

If they are supposed to be connected all day 9-5, you could also setup notifications to only email you during 9-5 if they are disconnected, so you wouldn't have to watch it, but just be alerted if someone loses connection.

EDIT:
As an easier way to configure your various hosts, services, dependencies, etc in nagios check out NConf. I had been managing a nagios server monitoring a good 50+ hosts modifying all the files via command line then switched to using NConf when deploying a new nagios server. You get a lot of power and options with NConf without really losing anything from the command line access.