The Polish government has made it clear that it will veto changes to the free movement of people in the EU Treaties. Since the EU Treaties can only be changed by unanimity, this is a blow to calls from Cameron's UK government (and from the Bavarian CSU) for changes to the system.
I've written before about how the UK debate seems to frame the single market as the only good thing about the EU when the social elements are such a big part of the bargain. This is desire for the single market to respect national social protections gives rise to a kind of European social contract - a kind of minimum (and from the left's point of view it is very minimum) level of protection that limits the single market in undercutting national welfare and social systems (though the single market has had a big impact on these). But now the debate has shifted to reviewing and limiting one of the fundamental freedoms of the single market.
The fears over EU immigration in the UK (with the ending of the restrictions on Romanian and Bulgarian citizens) has been the main spark for this, and the idea of being tough on immigration is popular. And because a "market" is seen as just goods you buy in a shop, rather than an economy people live in, the free movement of people is perhaps seen as not really being part of the single market. Despite this, there are polls indicating that Britain would be welcoming of immigrants who play by the rules, so the panic may be more linked to the rhetoric over benefits and the young than might be seen at first glance.
Watering down free movement rights is likely to come up against fierce opposition from many Member States - not just Poland, but also Spain, Ireland, Greece, and other countries afraid to see their citizens treated as second-class EU citizens - so it's unlikely to work. But watering down further the social side of the EU will make everyone more economically insecure and could further undermine support for free movement rights and solidarity in the wealthier Member States, and ultimately support for the single market.
For Cameron to be able to claim victory through renegotiation, he will have to get something big in the area of social policy now that so many other areas are sealed off (the UK is outside of Schengen, the Euro and has an opt in to justice legislation). The undermining of social rules and standards needs to be resisted.