I am wondering if it is possible to have RAID1 on an ext3 filesystem in an external usb drive with two disks, like on this

Western Digital 1Terabyte External USB HD?

I know they come working out the box with RAID1 with like fat32 or something like that, but I deal with very large files sometimes, 2 to 4+ gigabytes - maybe more, and fat32 does not support that. I also would not like ntfs as I can't make every linux system at work support ntfs.

So, is there a way to have ext3 fs RAID1 on a usb external HD with two disks? Or is this new territory as I haven't been able to find reliable information on the internet . . .

Using software RAID, you can RAID any block devices, even loopback files (which I've done). So, yes, you can do this. I was running software RAID5 on 4 external drives up until a couple of weeks ago. No problems.

so, for example, would a software RAID1 on the above mentioned Western Digital be visable to any similar Linux System?

All Linux systems with a 2.4 kernel or later support software RAID.