Which 8GB iPod touch do not support iOS 5?
I will attempt to straighten all this out. Apple has a nice tool to enter a serial number into a web site and it will look up what model you have for you.


http://support.apple.com/manuals/#


You can even paste your serial number after the # sign above rather than pasting it into the search field. This is generally all you need to determine which generation of device you are being sold or have been sold.

The simple answer for OS levels for a specific device is to trust iTunes to always update a device if it is capable of a newer OS than it detects: If you connect your iPod touch to iTunes on a Mac or PC and click on the "Check for Update" button, and it does not indicate that there is a new iOS update for your iPod touch, then your iPod touch is already running the most recent version of iOS that it can run. Thus, it cannot run iOS 5.1.

I own a 4th Generation 8GB iPod touch, model MC540LL, and it does indeed run iOS 5.1.1. So it is not true that there is no 8GB iPod touch that can run iOS 5.1.1.

It appears from MrDaniel's link to everymac.com that the 8GB iPod touch owned by @Harriz_shiddiq does not run anything higher than iOS 4.2.1. Here is further evidence:

According to the complete history of model releases of the iPod touch, there is no such thing as a 3rd Generation iPod touch in the 8GB size. The 3rd Generation of the iPod touch only exists in 32GB and 64GB sizes. You can see for yourself in the internet archive's way back machine by examining the treatment of the model comparisons at the bottom of this page. The tech specs make you choose between identically sized models (visually identical externally) to see the specs for the 8GB model for sale or the 32/64 GB model for sale. The latter ones allow for headphones with remotes - the former ones do not. As far as I can tell, this also is a nice way to tell apart 3rd gen and later iPod touch as they can all use headphones with Apple remotes. All 2nd generation devices can not.

The downloadable free app MacTracker provides a database of comprehensive specifications of all the iPod touch models along with their chronological release history. From using this app, I've found the following information.


September 2007: iPod touch 1st Generation in 8GB and 16GB sizes (iOS 3.1.3 maximum)
February 2008: iPod touch 1st Generation in 32GB (iOS 3.1.3 maximum)
September 2008: iPod touch 2nd Generation (MB model designation) in 16GB and 32GB  (iOS 4.2.1 maximum)
September 2009: iPod touch 3rd Generation in 32GB and 64GB (iOS 5.1.1 or latest)
September 2009: iPod touch 2nd Generation (MC model designation) in 8GB (iOS 4.2.1 maximum)
September 2010: iPod touch 4th Generation in 8GB, 32GB and 64GB (iOS 5.1.1 or latest)
October 2011: iPod touch 4th Generation in 8GB, 32GB and 64GB in white color (iOS 5.1.1 or latest)


I cannot find Apple model number MC086ID/A, reported by Harriz_shiddiq, within the MacTracker database. MacTracker reports that there were two Apple model numbers for the 2nd Generation 8GB iPod touch: MB528LL/A, released September 2008, and MC086LL/A, released in September 2009.