again, I'm assuming you mean vertically, as you might in a Lian-Li case.
I currently have 4 SATA drives mounted vertically in a Lian-Li PC-61 case. I will say that I have been experiencing an intermittent problem with one of them (a Seagate 'Cuda 200GB), making a clicking noise and being removed from my system. I'm not sure what the cause of this is, but I haven't ruled out the mounting.
That said, my other three drives, including my Raptor system drive all work fine. Also, I haven't heard of anyone else complaining about problems with hard drive mouting in the Lian-Li cases, so I would say that you probably won't have a problem. I'm guessing my problem is more likely either a bad hard drive or a faulty SATA cable.