I've just successfully tested and installed ESXi 3.5 U3 on an IBM x3250 Server - there was no info on this anywhere on the net, so decided to post here in case someone else is looking to do the same thing
Server itself isn't listed on the HCL or tested lists, however the RAID Controller and Network Cards are - server had no issues throughout the install, and is solid.
IBM x3250 (4364-I2A)
Xeon 3040 @ 1.86Ghz
4GB DDR2 RAM
2x 160GB 7200RPM SATA HDDs in RAID 1
LSI 1064E SAS/SATA RAID Controller
2x Broadcom NetXtreme BCM5721 Gigabit Ethernet Cards