Hey Whips. I'm in exactly the same situtation for the second year running. Last year I was offered a PhD in January, but turned it down. I then got worried about whether there'd be any more advertised. However, it seems that there is are even more PhDs advertised later on - I got one in April, although ended up not not doing that either.
So, two main points...
1. There are certainly a lot more PhDs to be advertised. I have a few friends doing PhDs, and none of them had got theirs sorted by this point in the year.
2. The above doesn't mean, however, that you'll find one you like more than what's currently on offer. That's the hard part - I guess it's a bit like playing blackjack - stick with an offer, or twist?