So I had my PC set up as a dual boot linux (Ubuntu 11.10) / Win 7 box. I was using burg as the bootloader. Everything was happy.

Then I needed another partition of Win 7 for a customer (I keep some spare space for things like this). I had the following set up:
sda1 - Windows 7
sda2 - Ubuntu 11.10
sda3 - swap
sda4 - Extended Partition
sda5 - ntfs data drive
unallocated space

So I expanded the logical partition to include the unallocated space. No problem. I created sda6 in the new space (using gparted), still no problem. I installed windows 7 there and all was happy, except, as expected, windows replaced the bootloader and only elected to see the two windows partitions.

Fine. I booted up a liveCD and reinstalled burg (update-burg, burg-install). However, it didn't detect the new windows 7 partition (sda6).

I thought maybe I needed to mount the partition, so (rather than wait for the livecd), I rebooted into the physical ubuntu install (sda2) and tried again, this time with sda6 mounted to /mnt. Still no dice.

Not sure what I'm missing, but burg / grub never discovers sda6. I'm sure I can get back into it with a recovery of windows, but then I'm back in the same state where I can't get to linux.

(the second linux image is just an older kernel I haven't cleaned up yet)

Would appreciate any ideas!

Nothing to do with burg (which I haven't tried).
The os-prober only looks for boot code - Windoze always installs its boot code to a primary partition. Regardless of where the rest of the code goes.

Fair enough. So I'm semi-hosed? I'd have to blow away the linux distro and install windows (the second) to a primary partition like sda2. Then I could reinstall linux to the extended partition? Then grub would be able to find it?

Maybe it's best at this point to get a second hard drive . Then I'd have some spare primary partitions.

Actually I think you might be fully hosed.
I don't think M$oft will allow you to install 2 version of Windows. Certainly not like you want.
A "dual-boot" (as Redmont defined it) was only allowed between two different Windoze versions. Two only note - not that that affects you directly in this case. And the more recent one had awareness of the previous versions, and would replace that bootloader with its own, and set-up a boot menu with two options.
So you might wind up with a Vista (primary) partition that actually contained Win7 boot code that pointed to the Win7 partition to handle the actual boot. You couldn't do it in the reverse order.
I suspect you will only ever be able to boot one of those Win7 partitions - even from a M$oft loader.

But I haven't tried. My interest in Windoze has diminished remarkedly, and Win8 might just convince me to give it away all together.

Well, quick update. Seems I should have "just tried it".

I did a bit more investigating about how the windows boot loader works, and, turns out, windows is perfectly happy chainloading to its own boot loader, which can see both versions of windows.

So "it just works". It's not all three OS's managed by burg, but burg can ask whether I want Linux or Windows, and if I choose Windows, I can then select (from the windows boot loader) which version I want.

Not optimal, but it works for this (hopefully temporary) configuration.

Yeah that is always how it has to be done - the M$oft loaders aren't multiboot compliant.

Good luck to you - like I said, I'm surprised you can get both Win7's, but if it works ...