There will be a jump in the difficulty when difficulty goes to 10.0. That's because none of the 9-chains will qualify for blocks and we have to start looking for 10+-chains. I've actually been working on a paper related to this. Right now it looks like 10.0 will be more difficult than 9.996 will be which means we could get stuck between 9.996 and 10.0 for a while.
So now we know before reaching 10.0 ony 1/256 (0.4%) 9-chains found qualify. At the same time there are ~3% number of 9-chains of 10-chains qualify. So it looks like going over the 10-chain boundary would cause an 0.004/(0.03+0.004) = 10% decrease of chance to find a block.
Of course as mikaelh pointed out this assumes a flat distribution of Ferma-test remainder that is used to calculate the fractional part of the difficulty.
You can actually see that happening before in my charts:
http://xpm.muuttuja.org/charts/If you look closely enough, the network block rate seems to have dropped when we went from 8.996 to 9.0. Of course we were using an older version of the mining algorithm back then which probably behaved slightly different.
From the chart the difficulty reached 8.996 at c.a. 15:10 on July 21, and reached 9 less than 1.5 hours later. I guess 10.0 will be reached before December.