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.[10] SiS's 760 and
964 chipsets also initially exhibited
this problem, though it can be
rectified with an updated SATA
controller ROM.[citation needed]
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.