Other developers always ask me this, and it is a very common question in the forums. Many people argue that there is no danger in read only access. So for future referance, here is the Microsoft article that describes the issue, and an extract from it. (My thanks to Shane Young who sent me the link lately and reminded me where is was).
"customers are strongly advised against direct access in a read-only manner to these databases unless Microsoft protocol documentation is followed exactly. Accessing these databases programmatically or manually could cause unexpected locking within Microsoft SQL Server that can result in overall performance problems."