juju-core appears to be a non-starter.

ubuntu@bysen:~$ juju -vvv bootstrap --constraints arch=arm -e maas
2013-10-01 21:12:17 INFO juju.environs.tools tools.go:82 filtering tools by released version
2013-10-01 21:12:17 INFO juju.environs.tools tools.go:29 reading tools with major version 1
2013-10-01 21:12:17 INFO juju.environs.tools tools.go:37 filtering tools by series: quantal
2013-10-01 21:12:17 INFO juju.environs.tools tools.go:40 filtering tools by architecture: arm
2013-10-01 21:12:34 ERROR juju.tools list.go:113 cannot match tools.Filter{Released:true, Number:version.Number{Major:0, Minor:0, Patch:0, Build:0}, Series:"quantal", Arch:"arm"}
2013-10-01 21:12:34 INFO juju.environs.sync sync.go:63 listing available tools
listing available tools
2013-10-01 21:12:35 INFO juju.environs.sync sync.go:76 found 32 tools
found 32 tools
2013-10-01 21:12:35 INFO juju.environs.sync sync.go:80 found 9 recent tools (version 1.14.1)
found 9 recent tools (version 1.14.1)
2013-10-01 21:12:35 INFO juju.environs.sync sync.go:86 listing target bucket
listing target bucket
2013-10-01 21:12:52 INFO juju.environs.sync sync.go:112 found 8 tools in target; 1 tools to be copied
found 8 tools in target; 1 tools to be copied
2013-10-01 21:12:52 INFO juju.environs.sync sync.go:132 copying 1.14.1-saucy-i386 from https://juju-dist.s3.amazonaws.com/tools/juju-1.14.1-saucy-i386.tgz
copying 1.14.1-saucy-i386 from https://juju-dist.s3.amazonaws.com/tools/juju-1.14.1-saucy-i386.tgz
2013-10-01 21:12:52 INFO juju.environs.sync sync.go:146 copying tools/juju-1.14.1-saucy-i386.tgz
copying tools/juju-1.14.1-saucy-i386.tgz
2013-10-01 21:13:04 INFO juju.environs.sync sync.go:159 downloaded tools/juju-1.14.1-saucy-i386.tgz (4081kB), uploading
downloaded tools/juju-1.14.1-saucy-i386.tgz (4081kB), uploading
2013-10-01 21:13:04 INFO juju.environs.sync sync.go:160 download 4081kB, uploading
download 4081kB, uploading
2013-10-01 21:13:05 INFO juju.environs.sync sync.go:117 copied 1 tools
copied 1 tools
2013-10-01 21:13:05 INFO juju.environs.tools tools.go:82 filtering tools by released version
2013-10-01 21:13:05 INFO juju.environs.tools tools.go:29 reading tools with major version 1
2013-10-01 21:13:05 INFO juju.environs.tools tools.go:37 filtering tools by series: quantal
2013-10-01 21:13:05 INFO juju.environs.tools tools.go:40 filtering tools by architecture: arm
2013-10-01 21:13:24 ERROR juju.tools list.go:113 cannot match tools.Filter{Released:true, Number:version.Number{Major:0, Minor:0, Patch:0, Build:0}, Series:"quantal", Arch:"arm"}
2013-10-01 21:13:24 ERROR juju supercommand.go:282 command failed: no matching tools available
error: no matching tools available

It's also doing something weird, complaining about a environment that isn't defined and doesn't exist.
Completely ignored "default:" in my .config too.

ubuntu@bysen:~$ juju -vvv bootstrap --constraints arch=arm
2013-10-01 21:14:30 ERROR juju supercommand.go:282 command failed: unknown environment "cabs-pts-cx-maas"
error: unknown environment "cabs-pts-cx-maas"

ubuntu@bysen:~$ cat ~/.juju/environments.yaml 
default: maas
environments:
 maas: # admin account
  type: maas
  maas-server: http://10.229.32.254:80/MAAS
  maas-oauth: QHrR3GugqLC8BvCuQU:DzHMamPBzs26gRuh7j:KE3LuTKAvCPLZXhjaAbK82nVUEq7r8Py
#  maas-oauth: DpGkmSKnKzGyJRW8QB:UUNw9cT2xh3rU8Fbq9:bMECDATsYLtATZzG5n8dsvr8X3jUyX49
  admin-secret: nothing
  default-series: quantal
  authorized-keys: |
   ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDcpgHwInDlU7Gs1oLhXAaGJk4Ihl8bdId5Ee/1uh2kEteB27sMUmuzCIZwNqa+mkJyKyp5Abk44bSZTHxRt/uZZBpFPTLTOI+YxwVKWnP4JcfyuWBOGNaNaJr9VmyuVsGJseZ76TyDB3yCQotwQct64/ydy6DWDffl2Wo6gvG8vQDj5b2PmBRfolMGZNan/a6oosOpsX2uZU0IZGgHBap/zkAwMM6Q1hd2vmsnolGXskGORX+/tOFTZP5tQnJ3deCMY+Nby/XKAHHEYca2jMktGXsPUlB0nCy6+ogl+CH1DgiWIrV2NKOXsEE9oWVf85JOawzA0rFC6DpN3+fQoB5/ ubuntu@bysen

Is that helpful?