How come after it rains some concrete walkways are wet while others are dry?

That would be my bet. I see it too.

I would agree. It has something to do with different types of cement. It might also have to do with relative ground temperature, shade/sunlight, airflow (near windy alleyway vs. next to building).

Ooo, and let me go for an obvious answer; some are dry because there is a canopy over them. XD

Yeah, I think your right though, that probably has something to do with it.

I was all set to be like cause some of them have trees directly above them.

But yeah, I’m gonna go with porous.

The way I see it, if you have X inches of water, it will sink to be Y inches from the bottom of the concrete. More concrete in a single location will mean that the water is farther from the surface making it look dry.

While concrete is naturally porous and hydrophilic, the phenomenon you see is due more to the hardness of the original mixture, resultant surface texture or finish, sealant used (or not), and age- which affects all of the above to some extent. Fun reading:
http://matse1.mse.uiuc.edu/concrete/prin.html
and:
http://www.kta.com/knowledge/PACE2006/WaterproofingConcrete.pdf
and:
http://www.toolbase.org/ToolbaseResources/level4FAQ.aspx?ContentDetailID=3547&BucketID=5&CategoryID=25
and a news story concerning an engineering explanation of sorts:
http://www.mlive.com/chronicle/news/index.ssf/2008/11/concrete_goof_results_in_rippe.html
something interesting:
http://www.media.rice.edu/media/NewsBot.asp?MODE=VIEW&ID=9515
and something else ineresting:
http://media.www.thetraveleronline.com/media/storage/paper688/news/2008/10/31/News/Slippery.Sidewalks.Pose.Problems.For.Ua.Campus-3518718.shtml

ok, I’m done

that is exactly what i was going to state depends on the concrete