OK, not sure if this has already been mentioned so if it has sorry. Here's the simple easy steps I have taken to get Cricket working. I've probably flashed ROMs about 40-50 times and can get Cricket fully working quickly. Between PPCGeeks and HoFo there is a lot of Cricket info floating around. It was a bit confusing at first and in hindsight I ended up doing some things manually and then unwittingly using cabs that did the same thing again.
These steps worked for me. I take no responsibility if you brick your TP and I take no credit as none of this is my work.
1) Make sure your phone is UNLOCKED. If you don't know what I'm talking about then you probably shouldn't be messing with MMS & WAP settings either. Just search around here and HoFo and you'll figure it out.
2) I used the following info from Ryan Mogul at HoFo for setting up my ePST.
http://www.howardforums.com/showthread.php?t=1507276 You should follow every step exactly. Go slow as some of it can be confusing and a simple mistake here will surely have you chasing all sorts of problems down the road. Also keep in mind that while these steps that Ryan has outlined can take a while, you only need to do it once. Doesn't matter if you flash 100 ROMs, these ePST settings will stay.
3) Flash any ROM you want to try
4) Do the MMS settings as Ivan kindly mentioned in Post #1 of this thread
5) That's it, you're done. WAP and MMS should be working, as should voice and SMS. Only thing that doesn't work is the email issue everyone has with Cricket. Spend the rest of your time restoring your contacts, setting up email accounts, loading cabs, customizing, tweaking....whatever.
*6) If you plan on flashing ROMs often, invest in a solid backup program. I use Sprite backup personally, but many others work well too. This way I can get the phone just how I like it, back it up, then play around with other ROM's and easily get back to my perfect setup that I know will work.
Oh, and one final bit of advice that has helped me. Create a text document and copy/paste all the steps you take to get your phone working. Any time you manually do a regedit that you like add it to your document. That way you will always have your very own guide to get your phone setup and won't have to hunt through all these long forum threads. I can't tell you how many times I knew there was a post that had some info I needed but I couldn't remember if it was here or at HoFo and which thread it was in.