I certainly noticed it against Wigan and even Wire (but i suppose they were just desperate), and it was also present against Hull, but the conditions could offer some excuse in this case. In actual fact it's not just holding down it's 'turtling' where the player is turned on their back once tackled.
I've been trying to see if we're doing it too and would have to say we're not. In fact we're jumping off probably a little too soon - presumably scared of the refs and how we got done last season. Reminds me of England in the World Cup, playing to a different set of rules.
In any case it won't come to anyone's attention but ours until either Saints, Wigan, Leeds or Wire complain about it so in the meantime let's just try gently reminding the referee of the rules whenever it occurs in our games.