I dont know why you guys are suggesting adapters and SATA DVD-ROMs.
There is no good reason to go and spend money on a SATA DVD-ROM or even a burner.
If more airflow is required, then you can get round IDE cables.
And, to answer the original question: You will not encounter any problems by adding an IDE CD/DVD-ROM/RW/etc while you use a SATA HD. In fact, I am on my computer now, which has a SATA HD and an IDE DVD-RW.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64