Currently, an issue can be assigned to any user who has the "Assignable User" permission, regardless of their current availability or workload. It would be useful to be able to check an assignee's availability both in terms of scheduled availability (i.e. are they in the office) and in terms of current workload.

As a workaround, if you make a transition that includes the Assignee field, it should be possible to write a condition or validator to control whom the issue can be assigned to:

http://confluence.atlassian.com/display/JIRA/Workflow+Plugin+Modules