I am a fraud investigator at a large national bank for my full time day job. We are gettin hit hard with these scams, sometimes we see 40-50 per day. Its amazing how people fall for the obvious. No, your girlfriend you just met online DOES NOT LOVE YOU and which you have never met is sending you $2000-5000 and asking for you to send it back via Western Union.
Keep in mind that these checks largely come from compromised business accounts which can take many months to return as lost/stolen/forged. If it were desposited it may be quite a while before it charges back, potentially leaving you in the negative and you are the one held responsible.
Criminals are slick and pretty smart. All day long I deal with these dating scams, the "you won the lotto" scams, inheritance from someone you didnt know existed scams, and the ever popular Craigslist scam. We have to build a case large enough for law enforcement to accept, which is typically $75k-100k for secret service and FBI, but the difficult thing is that these people have been traced back to West Africa which makes it a nasty international matter. Yuck
Sorry for rambling on, but this is a subject I can finally contribute something useful to