How safe do we need a bitcoin to be? Well, it depends on price.
Difficulty means that bitcoins are more secure. Mining is used to process transactions, and as we reach an increasing computing capacity it's more and more difficult to trick the system into double-spending. When price rises, miners back the value of a bitcoin making it safer.
In other words, it will only be a bubble if we don't actually need bitcoins to be that safe.
You are equating 'safe-ness' of the currency (ie can it be hacked, or otherwise fraudulently manipulated) with pure stampeding feeding-upon-itself bubblemania...two different issues.
Right now it is pretty clear that demand is overwhelming supply of BTC...the only question that needs answered regarding whether this is a bubble is if people are demanding those coins for the exchange of goods and services or are they demanding those coins because they just went up over %9000 and they are convinced they will do it endlessly...and so snatch up the meagre supply at that price and stash them in an encrypted wallet....i'm guessing it's the second...
The real test comes when some bit of news or outside disturbance of some kind causes a little hiccup in this run-up..and starts shaking a few speculators from their trees....(ie a correction).