Is it safe to seed a random number generator from system time? It seemed to me that the Bouncy Castle SecureRandom class for C#/.NET only uses DateTime.Now.Ticks as its seed by default.

I was wrong, but wouldn't that be unsafe to do?
Yes, it is unsafe to seed a PRNG with only with the system time.  No, that's not all Bouncy Castle's SecureRandom does.

The SecureRandom default constructor calls SetSeed(GetSeed(8)); which calls Master.GenerateSeed(length); which calls SetSeed(DateTime.Now.Ticks); which is misleading because SetSeed only adds seed material to an already existing prng (the static instance called Master).  If you look at the static declaration for private static SecureRandom Master you'll see that it actually seeds itself from a combination of Ticks and ThreadedSeedGenerator.

Why the system time is not enough

Using just the system time would be unsafe.  The whole point of a crypto random generator is to be unguessable by an adversary, and by using the system time as a seed to the random number generator, you make it easy for an adversary to guess your seed and hence your random output.

DateTime.Now.Ticks has precision of 10 million ticks per second.  Suppose an attacker knows what time you generated a key, +/- 1 second.  They only need to make 20 million guesses ($2^{20}$ or so) in order to guess your random seed.  This is likely crackable in a matter of seconds with a modern laptop.

Suppose an attacker has absolutely no idea what time you generated some key.  They only know it was after – let's say – the year 2000.  Since it's 2014 now, the entire space of 14 years occupies 51-52 bits.  log2(10000000*60*60*24*365*14) ~= 51.97…  This means the absolute highest level of entropy that you're likely to have in anything generated from system time is around 51 bits, which is small enough to be easily crackable, and realistically, you'll never even come close to that.