Shot peening of Al alloys is not very common in the automotive industry.  Al aerospace parts like aircraft wheels are routinely shot peened.  Cast steel shot or conditioned cut wire are used.  Ceramics may be used in some instances.  You can find a great deal of information on shot peening by using the following links:

http://www.shotpeening.org/ICSP/confrns.htm

http://www.shotpeener.com/

http://www.metalimprovement.com/

Thanks TVP.
It was very helpful.

Shot peening is described in MIL-S-13165C SHOT PEENING OF METAL PARTS (S/S BY SAE-AMS-S-13165) and MIL-P-81985 PEENING OF METALS.  Specifications for peening media and intensity are given for different substrate metals.  Cast steel shot or cut stainless steel wire may be used on Al (as mentioned by TVP).
http://assist1.daps.dla.mil/quicksearch/

Shot peening of Al alloys is commonly performed on aircraft parts to be hard anodized.  The shot peening induces residual compressive stresses to compensate for the decrease in fatigue life due to the relatively brittle anodic coating.  Common applications are hydraulic cylinders and pistons.  All shot peening residues are chemically removed prior to anodizing.  – MIL-S-13165C, paragraph 3.3.10.2; MIL-P-81985, paragraph 3.5.2.1.  Nitric acid-based solutions are used.

Shot peening can also eliminate anodizing defects due to the presence of primary solidification intermetallic phases.*  The shot peening breaks up the intermetallics within the near-surface Al matrix to a small enough size so as to not blister the subsequent hard anodizing, and is a common procedure for helicopter manufacturers. – personal discussion, Leah Markowitz, HR Textron, 1998 SUR/FIN conference.
*These can occur in diameters comparable to the hard anodizing thickness in castings and forgings of high strength alloys such as 2024 and 7075.  

Some useful literature articles:
“Effect of Shot Peening Variables on Fatigue of Aluminum Forgings,” Metal Progress, v. 120, No.2, p. 33-35 (1981) and
“Hard Anodic Coatings on Aluminum Alloys:  Evaluation and Control of Porosity,” Metal Finishing, p. 19-24 (December 1992).

we used to do shot peening on Aluminium Castings but here is a few  strange concepts

we used to peen swingarms for motorbikes and inlet manifolds for racing cars. Basically it was to give a good surface finish and remove any ....ahem suface defects.

on the other hand shot peening of Aerospace castings was a definate No-no. the reasoning beyond this is that you would peen over any surface defect and  hide them, despite the fact that they were being xrayed.

i would say in general its done for cosmetic reasons.