Mining profitability sounds too good, where is the catch? I am a beginner looking into Bitcoin mining. I don't want to waste time (or electricity) so I started looking into hardware mining rigs, for example:
Saturn

priced at $3000 USD, 
275GH/s, consumes ~400Watt
(or even Monarch from butterfly labs which claims 600GH/sec @ 350W, priced $4700)

I put these numbers + my local electricity price + current difficulty into a decent calculator, 
and I get >7,000 $USD profit for 3 months time frame (or >18,000$ for the Monarch). Sounds a bit too good to be true.

I know BTC/USD rate might fluctuate, and that the miner will produce heat, but still. - Where's the catch/mistake, if at all?
Keep in mind that difficulty also evolves over time: the more hashes the network produces, the higher the difficulty becomes.  As of this writing the difficulty is 148,819,199.  By my computation, each additional 275GH/s rig that joins the network will increase the difficulty by about 40,000; if 4,000 of them are sold, the difficulty will more than double (not counting all the competing mining rigs that may join the network).  And of course, in a few more months, there may be even more powerful rigs available.  So you should expect difficulty to rise, perhaps sharply, and this will decrease your profitability over time.