Do participants get time to think about their meal?

As far as I can determine, the contestants do not appear to have any "preparation time" between the moment they open the basket and the moment they start cooking.

For starters, the contestants aren't given any information before-hand about the contents of the basket. This is in contrast, for example, to Iron Chef, which gives the chefs a list of three possible "secret ingredients" so they can prepare 3 sets of dishes. Chopped throws the ingredients at the contestants and watches their reaction. According to Ted Allen, this is one of the key parts of the episode:

Do the chefs really have no idea ahead of time what ingredients are in the mystery baskets?

That is absolutely true. They do not know what the food is. We do, but they don't. We want them to be shocked. We've set it up very carefully so that when they open the baskets, we have all 10 cameras right in their faces to catch reactions from every angle we can. ^src

Rob Bleifer (Food Network's Chief Executive) echos that statement, though he does say the contestants are given a list of pantry ingredients to prepare for:

How much info, if any, do you provide the contestants about an ingredient? If parts are inedible, do you tell the chefs?

No. The only info they are given is the pantry list. They are given a full pantry list so they know what items exist — they don’t know where they are placed, but they know they are there. Anything else, we’re under the assumption that they should know. We would never put an ingredient in the basket that is toxic, etc. ^(src)

According to one contestant, the show only gives them a very short time to prepare between rounds before they're ready to start cooking: just long enough to dig through the pantry to find the stuff they need from the ingredients lists:

VC: On Chopped you have the mystery ingredients you need to use, but also the pantry for more common ingredients. Did they give you an inventory of what was in there or show you around?

JL: Yeah, they gave you about five or 10 minutes to walk around the pantry and familiarize yourself with everything. Actually they do this before each round, because the pantry ingredients change periodically between segments. ^src

Beyond that, the contestants have very little time to figure out what they're going to cook, because they're immediately on the clock:

AVC: What’s the process like for improvising a dish like this? In the first round your ingredients were a polenta log, tomatillos, braised beef brisket, and Araucana eggs [which are from a particular breed of chicken]. Those at least seem pretty familiar.

JL: I was lucky with that. I probably gave myself about 10 seconds to plan out what I was going to do each round, and the way I approached it was by taking the raw ingredients I wasn’t sure about, like the polenta and tomatillos in the first round, and figuring out what I was going to do with those first, if they were going to be a spread or some other kind of component. ^src

Each episode is taped during a single, continuous 16-hour filming period; if you factor in the time to cook, the time to present each dish, the time for the judges to finish eating and make their decision, the time to swap out pantry ingredients, etc., that doesn't leave much time left for the contestants to sit around planning things.