Its common for cheap batteries on ebay etc to lack the protection chips for over-charge, short-circuit, over-drain.
None of those are likely in your phone, but be careful if carrying them outside your phone.
And if using an external charger, check it does not overcharge. (4.2V max is normal). I bought one kit that charged to 4.4v - not enough to start a fire, but would significantly reduce the lifetime, and add some risk.
Some cheap batteries come close to original capacity spec's and some are awful.
I ordered a couple from DX. the reviews were good, but you never know there if the current batch will be as good as the last one.

edit: oops, didn't read page 2 of the thread. Like those guys above said ... :-)