Are you running OpenSolaris as a VM because you want ZFS? If so there are a couple options for Linux you could try.
One possible option has already been mentioned which is the ZFS FUSE project. I have used this in conjuction with ESXi and have found that when combining it with an iSCSI target it becomes a little too slow for really good performance. For me VMware complained a lot about the latency increasing between the server and the storage target.
Your other option is a project started by a group over at Lawrence Livermore National Labs. They are in the process of porting ZFS over to native Linux kernel modules. I have also used this after having issues with the FUSE equivilent.
I have to say its not quite complete but had all of the feauters I required (raid and dedup). Currently they are up to v0.6.0-RC9 which is based on ZFS pool v28 and FS v5.
I am not sure where OpenSolaris currently stands on ZFS version.
The benefit of running ZFS as native kernel module is you can access ZFS vdisks as native block devices inside of Linux meaning you can format the whole disk with ZFS instead of having to format it so you can store the VM and then having the vdisk on top of that. This way should provide better performance.
One disclaimer that I found after little research, ZFS on Linux is NOT BOOTABLE with out a little work. The link I have included is for Ubuntu which I have had sucess with in the past.
As far as what to do about VMs, you have a few option like you mentioned. I personally run an ESXi server and VMware Workstation. It really come down to what you plan to do with the VM, for example Multimedia will work better on VMware Workstation.
Straight virtualization such a hosting servers will work better on ESXi because it was pretty much built for that purpose and becasue of that has some picky hardware requirements. Not everything is supported by ESXi.
If you are using virtualization to achieve a multi-platform development environment then I would go with VMware Workstation it only $199 and $80 off if you are a student.
Hope this helps a little.