HTC Incredible says SMS storage is full, but it isn't
The Droid Incredible's available space for apps to store their data is only 150 MB, but it's impossible to view the amount of free app space on the stock ROM.  So the error message is correct, but you have no way to view the available free space.

In technical terms: /data/data is mounted on a different partition than /data, and all your free space views show you the amount of free space in the 750 MB /data partition.

What you need to do is start clearing data for spps that are taking up lots of space and that you don't use often.  One possibility is clearing the web browser's cache.  Facebook is another big data hog.  Check your other installed apps to see how much space they are using on data.

Here's a dump of the df -h output on my Inc:

Filesystem                Size      Used Available Use% Mounted on
tmpfs                   206.5M     32.0K    206.4M   0% /dev
tmpfs                   206.5M         0    206.5M   0% /mnt/asec
tmpfs                   206.5M         0    206.5M   0% /mnt/obb
/dev/block/mtdblock3    248.0M    121.8M    126.2M  49% /system
/dev/block/mmcblk0p1    748.0M    237.7M    472.3M  33% /data
/dev/block/mtdblock6    149.0M     79.7M     69.3M  54% /data/data
/dev/block/mmcblk0p2    193.7M     37.1M    146.5M  20% /cache
tmpfs                     8.0M         0      8.0M   0% /app-cache
/dev/block/vold/179:3     6.6G      1.2G      5.4G  18% /mnt/sdcard
/dev/block/vold/179:3     6.6G      1.2G      5.4G  18% /mnt/secure/asec
/dev/block/vold/179:9     1.8G    951.8M    930.1M  51% /mnt/emmc


(Note: I'm running a recent Cyanogenmod build that allows for swapping the SD Card and internal storage mount points -- that's why /mnt/sdcard is showing up as 6.6G while /mnt/emmc is only 1.8G.)

You can see how /data is a 750MB partition with plenty of free space, while /data/data is a teensy 150MB partition that's over half full.

This XDA Thread has a flashable mod that will make /data/data a subfolder of /data rather than a separate mount point, unlocking all that 750MB for app storage and allowing all your free space views to be accurate.  You'll need to root your phone in order for this to work since it also converts the filesystems to ext4 and that's going to require a replacement kernel.  (Recent Cyanogenmod builds already have this support, but if you're running the stock ROM you'll need to flash a different kernel.)