UPDATE: Looks like the cast is shooting in Chester at Harrah's.
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As originally reported by The Fightins last week, Chase Utley and Ryan Howard will be filming an episode of It's Always Sunny in Philadelphia on June 21st or 22nd (21st is an off-day, 22nd night game against Cleveland). No details on where it will be shot, but I'm guessing camera shy Chase Utley will be delightfully awkward/man sexy.
The cast of IAS will also be attending the grand opening of Mac's Tavern at 226 Market Street on June 23rd. Writer/actor Rob McElhenney and wife/co-star Kaitlin Olson are investors in the bar based on the fictional establishment.
And just cause, here is the letter Mac wrote to Utley last season: