I got this idea to test Eric Lubow’s ec2-consistent-snapshot With Mongo.
Using MongoDB 1.8.1 and Ubuntu Lucid AMI from www.alestic.com

On a running Ubuntu Lucid EC2 instance, connect using SSH and do the following:

1. Setup MongoDB. We stop it so we can move it’s files to the RAID volume.

echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' |   \
sudo tee /etc/apt/sources.list.d/10gen.list 
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10 

sudo apt-get update 
sudo apt-get -y install mongodb-10gen 
sudo /etc/init.d/mongodb stop 

2. Create 4 EBS volumes on the same zone of the instance and attached the volumes using the devices from /dev/sdh to /dev/sdk

3. Setup RAID0 volumes for MongoDb.

sudo apt-get install -y mdadm xfsprogs
sudo mdadm --create /dev/md0 --level 0 --chunk=256  \
  --metadata=1.1 --raid-devices=2 /dev/sdh /dev/sdi /dev/sdj /dev/sdk 

echo DEVICE /dev/sdh /dev/sdi /dev/sdj /dev/sdk  | sudo tee /etc/mdadm/mdadm.conf
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf

sudo mkfs.xfs /dev/md0
echo "/dev/md0 /raiddrive xfs noatime 0 0" | sudo tee -a /etc/fstab

sudo mkdir /raiddrive
sudo mount /raiddrive

sudo mv /var/lib/mongodb /raiddrive/
sudo mv /var/log/mongodb /raiddrive/mongodb-log

sudo mkdir /var/lib/mongodb 
sudo mkdir /var/log/mongodb 

echo "/raiddrive/mongodb /var/lib/mongodb none bind" | sudo tee -a /etc/fstab 
echo "/raiddrive/mongodb-log /var/log/mongodb none bind"  |   \
sudo tee -a /etc/fstab 

sudo mount /var/lib/mongodb 
sudo mount /var/lib/mongodb

sudo /etc/init.d/mongodb start 

4. Install Alestic ec2-consistent-snapshot

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv BE09C571
echo 'deb http://ppa.launchpad.net/alestic/ppa/ubuntu lucid main' |   \
sudo tee -a  /etc/apt/sources.list.d/alestic-ppa-lucid.list
sudo apt-get update 
sudo apt-get install -y ec2-consistent-snapshot

5. Use ec2-consistent-snapshot with MongoDB support which I modified a bit for Ubuntu. It’s on https://github.com/cocoy/ec2-consistent-snapshot.

sudo apt-get -y install build-essential libio-socket-ssl-perl libdatetime-perl 
sudo PERL_MM_USE_DEFAULT=1 cpan -fi MongoDB MongoDB::Admin

6. Copy this ec2-consistent-snapshot to /usr/bin/ replacing old file. And make it executable.

 sudo wget -q https://github.com/cocoy/ec2-consistent-snapshot/raw/master/ec2-consistent-snapshot \
   -O /usr/bin/ec2-consistent-snapshot
sudo chmod +x /usr/bin/ec2-consistent-snapshot

7. Run the snapshot.

export AMAZON_ACCESS_KEY_ID=youraccesskeyid
export AWS_SECRET_ACCESS_KEY=yoursecretkey
sudo ec2-consistent-snapshot --debug \
--mongo \
--xfs-filesystem=/raiddrive/ \ 
--region us-east-1 \
--description "MongoDB RAID snapshot $(date +'%Y-%m-%d %H:%M:%S')"  \
vol-xxxx vol-xxxxx vol-xxxxx vol-xxxxx

It should be resulting to something like:

ec2-consistent-snapshot: Using AWS access key: xxxxxxx
ec2-consistent-snapshot: Wed Jun  8 09:11:56 2011: mongo connect on localhost:27017
ec2-consistent-snapshot: Wed Jun  8 09:11:56 2011: locking mongo
ec2-consistent-snapshot: Wed Jun  8 09:11:56 2011: mongo locked
ec2-consistent-snapshot: Wed Jun  8 09:11:56 2011: sync
ec2-consistent-snapshot: Wed Jun  8 09:11:56 2011: xfs_freeze -f /raiddrive/
ec2-consistent-snapshot: Wed Jun  8 09:11:58 2011: create EC2 object
ec2-consistent-snapshot: Endpoint: https://ec2.us-east-1.amazonaws.com
ec2-consistent-snapshot: Wed Jun  8 09:11:58 2011: ec2-create-snapshot vol-xxxx
snap-xxxxx
ec2-consistent-snapshot: Wed Jun  8 09:11:58 2011: ec2-create-snapshot vol-xxxx
snap-xxxxx
...
ec2-consistent-snapshot: Wed Jun  8 09:11:58 2011: xfs_freeze -u /raiddrive/
ec2-consistent-snapshot: Wed Jun  8 09:11:58 2011: unlocking mongo
ec2-consistent-snapshot: Wed Jun  8 09:11:58 2011: mongo unlocked
ec2-consistent-snapshot: Wed Jun  8 09:11:58 2011: done

References:
ec2-consistent-snapshot With Mongo
EBS RAID volumes
https://launchpad.net/~alestic/+archive/ppa
MongoDB EC2 Backup and Restore