Setup:

library(plyr)
d <- ldply(setNames(nm=8:10), function(y) data.frame(x=y, y=y))
d

##   .id  x  y
## 1   8  8  8
## 2   9  9  9
## 3  10 10 10

After latest changes to plyr, the .id column is a factor:

summary(d)

##  .id          x              y       
##  8 :1   Min.   : 8.0   Min.   : 8.0  
##  9 :1   1st Qu.: 8.5   1st Qu.: 8.5  
##  10:1   Median : 9.0   Median : 9.0  
##         Mean   : 9.0   Mean   : 9.0  
##         3rd Qu.: 9.5   3rd Qu.: 9.5  
##         Max.   :10.0   Max.   :10.0

Doesn't work with facet_wrap:

library(ggplot2)
ggplot(d, aes(x=x, y=y)) + facet_grid(~.id) + geom_point()

Session info:

sessionInfo()

## R version 3.0.2 (2013-09-25)
## Platform: x86_64-pc-linux-gnu (64-bit)
## 
## locale:
##  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
##  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
##  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
##  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
##  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
## [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] ggplot2_0.9.3.1.99 plyr_1.8.0.99      knitr_1.5.15      
## 
## loaded via a namespace (and not attached):
##  [1] colorspace_1.2-4   dichromat_2.0-0    digest_0.6.4      
##  [4] evaluate_0.5.2     formatR_0.10.3     grid_3.0.2        
##  [7] gtable_0.1.2       labeling_0.2       MASS_7.3-29       
## [10] munsell_0.4.2      proto_0.3-10       RColorBrewer_1.0-5
## [13] Rcpp_0.10.6        reshape2_1.2.2     scales_0.2.3.99   
## [16] stringr_0.6.2      tools_3.0.2