This is still happening with 1.17.5: br0 is not up in the bootstrap node:

andreas@nsn7:~$ juju bootstrap -v
Flag --verbose is deprecated with the current meaning, use --show-log
2014-03-19 12:49:09 INFO juju.environs.bootstrap bootstrap.go:46 bootstrapping environment "scapestack"
2014-03-19 12:49:12 INFO juju.environs.tools tools.go:85 reading tools with major.minor version 1.17
2014-03-19 12:49:12 INFO juju.environs.tools tools.go:96 filtering tools by series: precise
2014-03-19 12:49:19 INFO juju.environs.bootstrap bootstrap.go:58 picked newest version: 1.17.5
Launching instance
2014-03-19 12:49:20 WARNING juju.provider.maas environ.go:192 picked arbitrary tools &{"1.17.5-precise-amd64" "https://streams.canonical.com/juju/tools/releases/juju-1.17.5-precise-amd64.tgz" "b070028dc9537327885bd3f8deb302f47122e5e0a5d3281fb1e6bb74cadcba1d" %!q(int64=5190393)}
 - /MAAS/api/1.0/nodes/node-ddcaf98c-ab86-11e3-8c77-2c59e54ace74/
Waiting for address
Attempting to connect to darby.scapestack:22
Attempting to connect to 10.96.1.3:22
2014-03-19 12:56:28 INFO juju.cloudinit.sshinit configure.go:39 Provisioning machine agent on ubuntu@10.96.1.3
Warning: Permanently added '10.96.1.3' (ECDSA) to the list of known hosts.
Logging to /var/log/cloud-init-output.log on remote host
Installing add-apt-repository
Adding apt repository: deb http://ubuntu-cloud.archive.canonical.com/ubuntu precise-updates/cloud-tools main
Running apt-get update
Running apt-get upgrade
Installing package: git
Installing package: cpu-checker
Installing package: bridge-utils
Installing package: rsyslog-gnutls
Installing package: --target-release 'precise-updates/cloud-tools' 'mongodb-server'
Fetching tools: curl -sSfw 'tools from %{url_effective} downloaded: HTTP %{http_code}; time %{time_total}s; size %{size_download} bytes; speed %{speed_download} bytes/s ' -o $bin/tools.tar.gz 'https://streams.canonical.com/juju/tools/releases/juju-1.17.5-precise-amd64.tgz'
Starting MongoDB server (juju-db)
Bootstrapping Juju machine agent
Starting Juju machine agent (jujud-machine-0)
2014-03-19 13:00:13 INFO juju.cmd supercommand.go:302 command finished
andreas@nsn7:~$ juju status
environment: scapestack
machines:
  "0":
    agent-state: started
    agent-version: 1.17.5
    dns-name: darby.scapestack
    instance-id: /MAAS/api/1.0/nodes/node-ddcaf98c-ab86-11e3-8c77-2c59e54ace74/
    series: precise
services: {}
andreas@nsn7:~$ juju ssh 0
ifconfig
Warning: Permanently added 'darby.scapestack,10.96.1.3' (ECDSA) to the list of known hosts.
Welcome to Ubuntu 12.04.4 LTS (GNU/Linux 3.2.0-54-generic x86_64)

 * Documentation:  https://help.ubuntu.com/

  System information as of Wed Mar 19 13:01:09 UTC 2014

  System load:  0.45               Processes:           101
  Usage of /:   0.2% of 916.89GB   Users logged in:     0
  Memory usage: 2%                 IP address for eth0: 10.96.1.3
  Swap usage:   0%

  Graph this data and manage this system at:
    https://landscape.canonical.com/

  Get cloud support with Ubuntu Advantage Cloud Guest:
    http://www.ubuntu.com/business/services/cloud

  Use Juju to deploy your cloud instances and workloads:
    https://juju.ubuntu.com/#cloud-precise

*** System restart required ***
ifconfig
ubuntu@darby:~$ ifconfig
eth0      Link encap:Ethernet  HWaddr 2c:59:e5:3b:01:40  
          inet addr:10.96.1.3  Bcast:10.96.7.255  Mask:255.255.248.0
          inet6 addr: fe80::2e59:e5ff:fe3b:140/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:115213 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4425 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:173197684 (173.1 MB)  TX bytes:345088 (345.0 KB)
          Memory:fbf00000-fc000000 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:1175 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1175 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:396606 (396.6 KB)  TX bytes:396606 (396.6 KB)