default.rb

execute "update-ubuntu" do
  command "apt-get update -y;apt-get upgrade -y"
  action :nothing
end

remote_file "/etc/apt/sources.list" do
  source "sources.list"
  mode "0755"
  owner "root"
  group "root"
  action :create
  notifies :run, resources(:execute => "update-ubuntu")
end

Relevant section of debug output:
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: Processing execute[update-ubuntu]
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: execute[update-ubuntu] using Chef::Provider::Execute
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: Doing nothing for execute[update-ubuntu]
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: Processing remote_file[/etc/apt/sources.list]
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: remote_file[/etc/apt/sources.list] using Chef::Provider::RemoteFile
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: Checking remote_file[/etc/apt/sources.list] for changes
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: generating cookbook url for url=sources.list, cookbook=:apt, type=files, node=node[mychefclient1]
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: generated cookbook url: cookbooks/apt/files?id=sources.list&platform=ubuntu&version=9.10&fqdn=mychefclient1&node_name=mychefclient1&checksum=682cad40f9c0bafa948c3812f07dbfe546993e5c612322eeccb21731cef4c03f
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: Downloading from server: cookbooks/apt/files?id=sources.list&platform=ubuntu&version=9.10&fqdn=mychefclient1&node_name=mychefclient1&checksum=682cad40f9c0bafa948c3812f07dbfe546993e5c612322eeccb21731cef4c03f
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: Sending HTTP Request via GET to /cookbooks/apt/files?id=sources.list&platform=ubuntu&version=9.10&fqdn=mychefclient1&node_name=mychefclient1&checksum=682cad40f9c0bafa948c3812f07dbfe546993e5c612322eeccb21731cef4c03f
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: /cookbooks/apt/files?id=sources.list&platform=ubuntu&version=9.10&fqdn=mychefclient1&node_name=mychefclient1&checksum=682cad40f9c0bafa948c3812f07dbfe546993e5c612322eeccb21731cef4c03f 100% done (495 of 495)
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: remote_file[/etc/apt/sources.list]: Checking for file existence of /etc/apt/sources.list
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: remote_file[/etc/apt/sources.list]: File exists at /etc/apt/sources.list
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: remote_file[/etc/apt/sources.list]: Target checksum: 682cad40f9c0bafa948c3812f07dbfe546993e5c612322eeccb21731cef4c03f
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: remote_file[/etc/apt/sources.list]: Source checksum: 10adacf4ac0fdb18d8b3f4d6c7ef71a6b51ff3790c8ad21e149cf08496ea9ff4
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: remote_file[/etc/apt/sources.list]: checksum changed from 682cad40f9c0bafa948c3812f07dbfe546993e5c612322eeccb21731cef4c03f to 10adacf4ac0fdb18d8b3f4d6c7ef71a6b51ff3790c8ad21e149cf08496ea9ff4
[Wed, 02 Dec 2009 15:27:22 +0000] INFO: remote_file[/etc/apt/sources.list]: Updating /etc/apt/sources.list
[Wed, 02 Dec 2009 15:27:22 +0000] INFO: Backing up remote_file[/etc/apt/sources.list] to /etc/apt/sources.list.chef-20091202152722
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: Processing package[postfix]
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: package[postfix] using Chef::Provider::Package::Apt
[Wed, 02 Dec 2009 15:27:22 +0000] DEBUG: Checking apt-cache policy for postfix