According to the documentation on Dell's website (link), SAS 5iR does support SATA, but doesn't explicitly say SATA I or II. Kyle Smith is right in terms of speed negotiation. Newer controllers should be able to handle speed negotiation. I'm willing to bet older controllers might require a jumper to enable it.
To answer your question: I don't think anyone knows for sure unless they've run your exact setup with the 5iR and the Seagate Barracuda 7200.9. If I were a gambler, I'd personally take the chance as SATA is fairly mature and commonplace these days.
According to Wikipedia (link), the section on "SATA 3 Gbit/s (Second generation)":
Given the importance of backward
compatibility between SATA 1.5 Gbit/s
controllers and SATA 3 Gbit/s devices,
SATA 3 Gbit/s autonegotiation sequence
is designed to fall back to SATA 1.5
Gbit/s speed when in communication
with such devices. In practice, some
older SATA controllers do not properly
implement SATA speed negotiation.
Affected systems require the user to
set the SATA 3 Gbit/s peripherals to
1.5 Gbit/s mode, generally through the use of a jumper, however some drives
lack this jumper. Chipsets known to
have this fault include the VIA VT8237
and VT8237R southbridges, and the VIA
VT6420, VT6421A and VT6421L standalone
SATA controllers. SiS's 760 and
964 chipsets also initially exhibited
this problem, though it can be
rectified with an updated SATA
controller ROM.
Seeing as you're using the SAS 5iR and don't have to worry about the VIA/SiS chipsets, I'd be willing to try it out. Just my two cents.