Simply put EOL means no patches no support. From http://support.microsoft.com/lifecycle/?LN=en-gb&C2=1173 "Support ends 24 months after the next service pack releases or at the end of the product's support lifecycle, whichever comes first. For more information, please see the service pack policy..."
For service packs:
"Security updates released with bulletins from the Microsoft Security Response Center will be reviewed and built for the supported service packs only. Daylight Savings Time and Time Zone updates are built for fully supported service packs only."
XP SP2 is EOL
XP SP3 has a supported end date of 4/21/2010 that is when mainstream support ended. XP is in the extended support phase, so only security fixes will be available until 2014 (if I read the chart correctly). As always it's best to get information like this from the vendor.
As far as impact, it's a cost to upgrade so businesses need to evaluate whether or not to upgrade. Many companies have realized that newer machines come with win 7 licenses so there is no upgrade price.