As you're all hopefully aware, Chelsea have been awarded a top seed (or pot, as UEFA likes to call it) in this year's Champions League for the fourth season in a row. I was a little curious as to how well the seedings corresponded with the reality of the group stages, so I went back through each tournament until 2005 and grabbed pots vs. final group standings. The result was fairly unsurprising:
Figure 1: Champions League seeding vs. group stage standings, 2005/2006-2009/2010.
The moral of the story is that being in Pot 1 is a pretty good deal. In fact, only twice has a team in that pot failed to make it out of the group stages, Liverpool last year finishing third in Group E and Manchester United placing 4th in Group D in the 2005/2006 campaign, an event I still remember extremely fondly. Out of 40 teams, that's a failure rate of 5%, although that number jumps up if you just look at English Pot 1 teams. It's still pretty favourable to Chelsea no matter how you slice it.
In general, the top two teams in the seedings take the group, with far fiercer competition reserved for the Europa League spot that comes with a third place finish. It's been five minutes since I looked and I've already forgotten how many times a team that was seeded fourth has qualified, but the number is fairly uninspiring. 3rd seeds often place second (occasionally 1st), but they have a habit of performing like glorified fourth-potters; on one occasion they actually did worse than their supposedly inferior brethren.
Anyway, the point of the exercise is to show you that it's overwhelmingly probable that Chelsea will emerge from whatever group they find themselves in, and it's pretty likely, although by no means a lock, that they'll finish at the top of their group. Things don't get too stressful until much later on in the year.