1) OS 3.0 Preview: According to MecuryNews.com Apple is hosting an invite only "sneak peak" media event to show off version 3.0 of the iPhone OS and discuss some SDK updates on Tuesday, March 17. Precedent has been that New OS = New Handset.
2) Carrier Pricing: Newsfactor.com is saying that T-Mobile in Germany lowered iPhone prices in January and now rumor has it that O2 UK will do the same in short order (perhaps May)...the hopeful think the operators are clearing their shelves for new inventory.
3) Contract Cycles: As ChannelWeb points out many first gen iPhone users' contracts are coming up for renewal in June. Many believe Apple will want to have a new device ready for loyalists.
4) Palm Pre: The Guardian and DaringFireball.net suggest that Apple will want a device with features to address some innovations featured on the super-hyped Palm Pre, which is scheduled to be released on Sprint in May/June.