Thanks everyone for your input. I don't have an issue with the age alone because my kids use them in moderation, they have to ask first, and they don't bring them out of the house unless we're going on vacation or I have to take one to a doctors appt and the other has to wait, etc. He's very capable and does fine on it, and also plays fine with non-electronics for most of the day! Though if I gave him my iPhone and a DSi I'm sure he'd happily entertain himself for an hour

. We are an electronics family, we have iPhones (with many kids apps--electronic books, etc), a Wii, 2 computers, my dd has an MP3 player (not an Ipod), etc.
I have decided to wait though for a couple reasons. Going into the warmer weather I know we'll be much busier and he won't have the time to want to play very often. And as it gets nicer I won't let them play very often, so as a pp pointed out, I'll be getting more asking if he can play than I want until its newness fades! Also, $$ is a good reason! He most likely will get one for Christmas if that is what he continues to ask for. I definitely will keep in mind that GameStop will probably have more DSi's (he wants a blue one!) now that the XL is out. The other thing is his birthday and Christmas are fairly close together, so I'm often running out of ideas as all the family is asking what to get him as well.
I will keep an eye on Ebay to see if I can pick up a used Mario game for our GBA though, since i believe this is the real draw of the DSi for him. He loves Mario, we have Wii nights w/friends of ours from time to time and all have a blast playing it. We bought him a new bed last week and are breaking him from cosleeping and last night for the first time he didn't come to our bed at all, he was so proud of himself this morning and said he thought he needed a special treat--I was all ready for him to say DSi, but he just asked for a Hershey Kiss , whew! LOL! We'll be onto potty training next!