[Acer Iconia Tab A100] eww245:
For anyone that doesn't want to root the hard way crossix has come up with a double click root for Windows xdaforums.com/showpost.php?p=23052186&postcount=105

Update 2/26/12

/system can now be mounted writable see the bottom of this post.

So the old Honeycomb exploit has now been patched in ICS. But there was an exploit found in the newer ICS kernels. Written by saurik,: called mempodroid

There is an offset needed as an argument to the binary, for the a100 we'll use what has worked for the a200 as noted in sauriks github linked above.

The issue with this is mounting /system as writable. I'm not sure if it's something in ICS, but it appears to be write protected. As noted here and here we will loop mount the system partition.

The tools needed are:

1. mempodroid under Usage Instructions, download pre-compiled

2. busybox 1.20 snapshot 3-10-12

3. su the latest from androidsu.com, extract from system/bin

4. mount.txt script

After downloading and extracting place them all in a folder called tools.

This must be done with adb. Issue the following from cmd or a terminal:

$ adb shell mkdir /data/local/tools
$ adb push tools /data/local/tools ; adb shell
$ cd /data/local ; chmod 755 tools/*
$ cd tools ; ./mempodroid 0xd9f0 0xaf47 sh

If all went well you should be at a hash # prompt. This is temp root.

mount /system rw the new way:

# PATH=$PWD:$PATH
# sh mount.txt -o remount,rw /system

Copy su and busybox to /system

# ./busybox cp busybox /system/xbin; ./busybox cp su /system/xbin/
# chmod 6755 /system/xbin/su

Install busybox

# cd /system/xbin
# for i in $(busybox --list); do ln -s busybox $i; done; sync

Copy the mount script

If busybox is updated this step must be run again

# cp /data/local/tools/mount.txt /system/bin/mount
# cp /data/local/tools/mount.txt /system/xbin/mount

Done your a100 should be rooted

the old way:

Now lets loop mount /system

[b]This is no longer needed[/b]
# ./busybox losetup -o $((512 * 51200)) /dev/block/loop7 /dev/block/mmcblk0

# ./busybox losetup /dev/block/loop7 /dev/block/mmcblk0p3
# mkdir loop ; mount -t ext4 /dev/block/loop7 loop

Copy su and busybox to the new mount point.

# ./busybox cp su loop/xbin/ ; ./busybox cp busybox loop/xbin/
# chmod 6755 loop/xbin/su ; sync

If it worked your a100 is fully rooted. Make sure to install SuperUser from the Market.

Either get busybox installer from the market, and install it to /data/local/tools/loop/xbin

Or:

# cd loop/xbin
# for i in $(busybox --list); do ln -s busybox $i; done; sync

The mount point won't survive a reboot so in order to write to /system again run:

# busybox losetup /dev/block/loop7 /dev/block/mmcblk0p3
# mount -t ext4 /dev/block/loop7 /data/local/tools/loop

[update 2/26/12]

To mount /system as writable do the following from adb. We'll just make a directory called /data/loop for easy access.

$ adb shell
$ su
# stop
[b]your screen will go black[/b]
# mkdir /data/loop
[b]skip this if the loop is already set up
# busybox losetup /dev/block/loop7 /dev/block/mmcblk0p3[/b]
# mount -t ext4 /dev/block/loop7 /data/loop
# mount -o bind /data/loop /system
# start

You can write to /system with any app but /system can't be remounted ro then back to rw.

This can be added to /etc/install-recovery.sh to make it permanent

busybox losetup /dev/block/loop7 /dev/block/mmcblk0p3
mount /dev/block/loop7 /data/loop
mount -o bind /data/loop /system

Thanks to crossix as the first to get temp root, and Icewyng for pointing out the exploit and helping with the magic number.