I have successfully installed Sarge on this machine, althought it was not without problems
This machine has integrated 6-port SATA controller and 6 hotplug SATA bays. I was told from the reseller that althought it has hotplug bays, the controller is not hotplug-aware and for true hotplug I would need to buy 8-port HP SAS/SATA adapter. However my tests proved that IT IS HOTPLUG aware. I didn't save the generated logs, but the driver noticed the drive is not accessible and after waiting some secs it put it in "removed" state, so it disappeared. Also the linux raid noticed this and switched into degraded mode. When I later pushed the drive back, system immediately noticed new drive is here, set up timeout for it to spin and initialize it. Although before removal it was /dev/sdb and after it was /dev/sdc (but only until system restart).

OK, so now the installation.
SATA must be switched to Enhanced mode in BIOS and alho AHCI mode set to Enabled. I don't know if it works otherwise. AHCI mode can be enabled only when RAID is Disabled in BIOS, but that's no problem, because I don't think it is a true hardware RAID.

Support for AHCI SATA was added I think in 2.6.9 kernel, so Sarge install CDs with 2.6.8 kernels are out of luck. What I did was I burned a netinst image of unstable install (etch).  It has I think 2.6.11 kernel which recognized the controller without problems. When installing from this netinst CD you can choose which "version" to install - stable/unstable/testing. Installer will ask us what kernel we want to use. But when installing sarge, we can choose only from sarge kernel packages - those are 2.6.8 and don't support our controller. So choose any of them and complete installation.

Just before reboot, we have to get newer kernel. There are maybe better ways of doing this, but did it like this:
1) Downloaded newest 2.6 kernel (2.6.18.2) to another machine, configured it for ML150 (AHCI, IPMI, tg3 for network card...)
2) run "make deb-pkg" (maybe "official" debian way of making kernel also works, but maybe it has dependences that cannot be met on half-installed new machine)
3) From the installer choose menu item which looks like "Load additorial installer modules" and install ssh-client
4) scp the created kernel package to /target/root (I don't remember, but maybe also wget is working in installer)
5) ensure all filesystems are mounted under /target
6) run "chroot /target"
7) run "dpkg -i /root/kernel-image.....deb" (not /target/root, we are chrooted now)
modify LILO/GRUB configuration to use this new kernel
9) reboot
10) watch if system detects all hardware
11) if not run automatically, run "base-config" to finish the installation

This server has a support for IPMI (Intelligent Platform Management Interface), so you can get various information from it by using ipmitool (also as a debian package). For example the sensors data can be read. I don't know if this machine has also other sensors chip, but I suppose not.
I tried to get lm-sensors working, but without success. There is ipmisensors patch for 2.6 kernels (http://bmcsensors-26.sourceforge.net/). What it does is it periodically communicates with IPMI chip and puts sensors data in files under /sys filesystem. It can then be read by any process. However I didn't get the "sensors" command working with this module
Another problem is that communication with IPMI chip is really slow (like 20 seconds to get all the sensors data), I suppose it is because it is running in polled mode. It also uses about 20% of cpu during that time (reported as sys cpu). During boot I get this error:
ipmi_si: Trying SMBIOS-specified KCS state machine at I/O address 0xca2, slave address 0x20, irq 192
ipmi_si: ipmi_si unable to claim interrupt 192, running polled
ipmi: Found new BMC (man_id: 0x000000,  prod_id: 0x0000, dev_id: 0x01)
IPMI KCS interface initialized
ipmi_si: Trying ACPI-specified KCS state machine at memory address 0xca2, slave address 0x0, irq 0
Could not set up I/O space

Does anyone know if it is possible to enable the interrupt-driven mode?
However I have found a plugin to munin, which directly uses the ipmitool, you can find it here http://svn.noreply.org/cgi-bin/viewcvs.cgi/trunk/munin/ipmi_sensor_?rev=127&root=WeaselUtils&view=auto