The answer to this is quite complicated because if you think about sitewide linkexchanges, you figure out that multiple backlinks from same domain can`t hurt your rankings...
Also, there are advertisers all over the forums who pay reputated members for their signature backlink.
Even tho there are no proofs that this will get you a SE penalty, From my opinion, this is quite risky and i feel that one indexed backlink from this web 2.0 websites should be enough...
However if you want to go for this, you should keep us in touch with your results too.