Haven't found the relevant info yet as it applies to Wisconsin, what I did find is the membership form of the California Teachers Association:
http://www.ostahome.org/CTA_Docs/CTA Membership Enrollment Form.pdf
Again, not sure how any particular WI public unions work yet, but in the CTA you don't actually get that money back, it just "remains in the general fund". In theory this is the
only money that must remain in the general fund.. Say 200 teachers pay dues and 50% of each teacher's contribution goes to the PAC, if 100 of them opt out the union needn't decrease its PAC transfers at all so long as the opt-out group's total dues are less than or equal to the entire general fund. Effectively the opt-outs are meaningless unless they exceed the total non-PAC spending that would occur either way.
If the opt-outs aren't pretty significant it could make an artful end-run around the Beck decision.. Well, for the CTA at least, no time to dig around more right now.
*Edit: How about a system where if the union spends 20% of it's total dues collection on PACs and you opt out, your dues are actually 20% less? Maybe some unions do work that way, but in the ones that don't I still consider the PAC donations quite mandatory (and therefore taxpayer funded) unless the members opt-out en masse (probably doesn't happen much if ever since they don't even get the money back).