I've always found that coming with a theme for a groom's cake is one of the most fun parts of planning a wedding. It's a chance to be wacky and whimsical. Since it seems like Vegas themes have been extremely popular lately, it would be only natural to find inspiration for a poker-themed groom's cake.
I found two very cool ones below:
via Pink Cake Box - be sure to check out her cake blog!
There are other themes that you can incorporate if you don't want to look like a complete high-roller. How about using a giant-sized rolling dice, a skyline of Vegas, a slot machine, or the welcome sign to vegas? A big giant gold coin cake would also make a statement. Hmm. Perhaps too much of a statement, but I think it would certainly top the whimsical list.