The nasty memory leak in NetworkManager, existing since gutsy times I believe, is finally fixed in the intrepid packages. The bug fixed version 0.6.6-0ubuntu7 is not yet uploaded it seems, but should appear at the intrepid packages link soon.
Correction: The packages seem to be uploaded, but not shown on the linked webpage yet. So just replace ubuntu6 with ubuntu7 and can download it right now.
I found the files at http://ubuntu.interlegis.gov.br/ubuntu/pool/main/n/network-manager/.
Just download them and then install them with e.g.
sudo dpkg -i network-manager_0.6.6-0ubuntu7_i386.deb
That fixed the problem just fine for me.