In short, L3 enforcement is through a Juniper firewall (either ScreenOS or Junos based). It enforces network access at layer 3(IP Layer).
Layer 2 enforcment is done at the switch or wireless access point level and uses the 802.1X authentiction protocol suite to authenticate a user BEFORE they get any network access.
When will pulse be available on Mac OS X? It's been asked for and talked about for over a year. It's also been alluded to that it's coming soon. Is there any roadmap as to when soon is? Having to roll out different clients for Windows and mac for VPN, UAC, etc is a major pain, as is supporting them all.
Mac OS X 10.6.8 and Mac OS X 10.7.3 is supported in next major release that is 4.2.Kindly reach out to your Juniper team for more specifications on the feature as well as release dates.
Hope that helps