This topic was covered recently in this thread:
You have two choices. Either dual-boot the GRUB way, or the eComStation way. If you choose the eComStation way, El Vato gives a very nice step-by-step with screenshots in the above-mentioned thread. For your situation, though, there is no need for any special disks, as eComStation 1.x is already LVM-enabled.
For a step-by-step on the GRUB way, it is safe to consider eCS booting functionally equivalent to Windows. I have never installed Ubuntu, but from what I understand of it I would expect it to have a supported and documented method of setting up dual-boot with Windows using GRUB, and you could follow that to the letter and it should work with eComStation.
Let me give a brief rundown of the science involved, as it can help reduce error.
Windows, DOS, and OS2/eCS load themselves from the Boot Sector of their partition. The computer boots by first reading the Master Boot Record or MBR, on the drive. A standard MBR, as would be installed by one of these OS's, when executed, looks for the Active partition. That's just a flag on the partition table. It "boots" that partition by reading its Boot Sector, and voila, the OS installed there comes up.
(Actually Windows does some weird thing to try to kill other OS's so its MBR isn't really standard - but if it is the only OS on the drive it works the same for the purpose of this discussion).
To dual boot these, IBM provided the Boot Manager Roger mentioned. It comes with OS/2 and eComStation, but not Windows or DOS. It gets installed on a tiny special partition, which must be a primary partition, and when booted, it brings up a menu (which you configure at installation) to select which of the other partitions it should boot. Convenient! Note the Boot Manager must be the active partition for the standard MBR to boot it.
Linux cannot load itself from its boot sector, considering that the job of a bootloader such as GRUB. The Linux kernel is saved on a /boot partition, which is usually a small (<100mb) ext2-formatted partition containing the Linux kernel and some other stuff for boot.
When you install Ubuntu, or some other Linux distro, by default it installs the GRUB MBR, which is different from the standard MBR. When installed, it is programmed with a location of its binaries and config file, usually on that /boot partition. It does not care what is active, only where it should find its stuff. GRUB's menu is defined in its plain-text grub.conf file. Each entry defines an action, which can be a number of things. To load Linux, it needs a location of a kernel, any parameters to pass to it, such as where to find the root ("/") partition, and some other things. Optionally, it can "chainload" a partition's boot sector, which - you guessed it - is how Windows or eComStation would be loaded by GRUB.
So to dual-boot the GRUB way, you define your eComStation partition in the grub.conf, and you're done. This might even be possible within the Ubuntu installer, making life easier. Then you don't need the Boot Manager partition.
If you do use the eCS Boot Manager, you need the standard MBR, so you must not let the Ubuntu installer install GRUB's MBR over it. But then, (uh-oh) Linux can't boot! To solve this problem, you install GRUB instead on the boot sector of the /boot partition. Note the distinction, and pay attention to the way Linux names drives and partitions (and GRUB's way, which is different), and the installation will go nicely.
Now the /boot partition can be added to the Boot Manager's menu, and when it boots it, GRUB will load, and the grub.conf only needs to deal with booting Linux. I would caution you strongly, however, do not boot to the /boot partition using the Boot Manager before you install GRUB on it. Without an installed boot sector, there is just randomness there, and results could be destructive (spoken from experience).
I do hope this is helpful, rather than confusing.
I think I would use the tools in the eCS installer to partition the eCS partitions, leaving some freespace for Linux, and use the Ubuntu installer to partition that freespace for Ubuntu. Of course, either installer could do the work for both, and Roger mentioned some good tools, too, but doing it this way lets the Ubuntu installer guide you.