I've had no issues fitting some non-ESU stay alive units to Loksound decoders, so my experience echos that of Richard above.
The quote on the DCCguy website doesn't state it will invalidate the warranty, but specifically says "in the future the company may have to disallow warranty replacements for decoders damaged in this way". I cannot find anything on the ESU website to indicate they have implemented any change to the generous warranty they have in place. In fact the opposite exists, in that the Loksound manuals (incl. the Loksound 5) still show how to fit a non-ESU stay alive unit.
It will be interesting to see if anyone can provide a first hand experience of a decoder being damaged by a stay alive unit.
There's some great advice here: http://www.sbs4dcc.com/tutorialstipstricks/esuloksoundv40andselectkeepalive.html
Also do check the decoder manual, available for download from the ESU website.