In Modelsim-XE 62.g (ISE 9.2), Systemverilog simulation was broken. (It worked in the previous release, XE 6.2c)
But it's working again in XE 6.3c. XE's language-support is identical to Modelsim-PE -- it has most/all of the design-constructs, and a few verification constructs. Assertions, coverage, clocking-blocks, virtual interfaces supposedly aren't supported...
I found that the compiler compiles them just fine. Of course, the simulator prints a warning message saying these features are disabled in Modelsim XE.
You can also download Aldec ActiveHDL 7.3 SP1. It's less complete than Modelsim/XE/PE (no packages, many other limitations), but it supports SVA assertions (something XE/PE doesn't.) The downside is the eval-version only works 21-days, then it stops working.