The CS6 upgrade has been available to buy for over a year and will remain on sale for the foreseeable future after the Cloud moves on to the CC versions next week.
CS6 upgrade path is just from CS5. If you have Photoshop version 5 you can not upgrade you must buy full CS6 $699 in the US. http://www.adobe.com/products/catalog/cs6._sl_id-contentfilter_sl_catalog_sl_software_sl_c reativesuite6.html?promoid=KFPRM
If you select upgrade you will see CS5 or CS5 Extended is required.